diff --git a/src/parser/declarations/assertion_declarations.rs b/src/parser/declarations/assertion_declarations.rs
index 3a98563..0b0d16f 100644
--- a/src/parser/declarations/assertion_declarations.rs
+++ b/src/parser/declarations/assertion_declarations.rs
@@ -172,7 +172,7 @@ pub struct PropertyPortItem<'a> {
     pub nodes: (
         Vec<AttributeInstance<'a>>,
         Option<(Local<'a>, Option<PropertyLvarPortDirection<'a>>)>,
-        PropertyFormalType<'a>,
+        Option<PropertyFormalType<'a>>,
         FormalPortIdentifier<'a>,
         Vec<VariableDimension<'a>>,
         Option<(Symbol<'a>, PropertyActualArg<'a>)>,
@@ -472,7 +472,7 @@ pub struct SequencePortItem<'a> {
     pub nodes: (
         Vec<AttributeInstance<'a>>,
         Option<(Local<'a>, Option<SequenceLvarPortDirection<'a>>)>,
-        SequenceFormalType<'a>,
+        Option<SequenceFormalType<'a>>,
         FormalPortIdentifier<'a>,
         Vec<VariableDimension<'a>>,
         Option<(Symbol<'a>, SequenceActualArg<'a>)>,
@@ -975,11 +975,11 @@ pub fn property_port_list(s: Span) -> IResult<Span, PropertyPortList> {
     Ok((s, PropertyPortList { nodes: (a,) }))
 }
 
-#[parser]
+#[parser(Ambiguous)]
 pub fn property_port_item(s: Span) -> IResult<Span, PropertyPortItem> {
     let (s, a) = many0(attribute_instance)(s)?;
     let (s, b) = opt(pair(local, opt(property_lvar_port_direction)))(s)?;
-    let (s, c) = property_formal_type(s)?;
+    let (s, c) = ambiguous_opt(property_formal_type)(s)?;
     let (s, d) = formal_port_identifier(s)?;
     let (s, e) = many0(variable_dimension)(s)?;
     let (s, f) = opt(pair(symbol("="), property_actual_arg))(s)?;
@@ -1448,11 +1448,11 @@ pub fn sequence_port_list(s: Span) -> IResult<Span, SequencePortList> {
     Ok((s, SequencePortList { nodes: (a,) }))
 }
 
-#[parser]
+#[parser(Ambiguous)]
 pub fn sequence_port_item(s: Span) -> IResult<Span, SequencePortItem> {
     let (s, a) = many0(attribute_instance)(s)?;
     let (s, b) = opt(pair(local, opt(sequence_lvar_port_direction)))(s)?;
-    let (s, c) = sequence_formal_type(s)?;
+    let (s, c) = ambiguous_opt(sequence_formal_type)(s)?;
     let (s, d) = formal_port_identifier(s)?;
     let (s, e) = many0(variable_dimension)(s)?;
     let (s, f) = opt(pair(symbol("="), sequence_actual_arg))(s)?;
diff --git a/src/parser/declarations/covergroup_declarations.rs b/src/parser/declarations/covergroup_declarations.rs
index 0736eb7..00df2f8 100644
--- a/src/parser/declarations/covergroup_declarations.rs
+++ b/src/parser/declarations/covergroup_declarations.rs
@@ -715,10 +715,10 @@ pub fn hierarchical_identifier_or_class_scope(
     ))(s)
 }
 
-#[parser]
+#[parser(Ambiguous)]
 pub fn cover_point(s: Span) -> IResult<Span, CoverPoint> {
     let (s, a) = opt(triple(
-        opt(data_type_or_implicit),
+        ambiguous_opt(data_type_or_implicit),
         cover_point_identifier,
         symbol(":"),
     ))(s)?;
diff --git a/src/parser/declarations/function_declarations.rs b/src/parser/declarations/function_declarations.rs
index 8ce24dd..1be3968 100644
--- a/src/parser/declarations/function_declarations.rs
+++ b/src/parser/declarations/function_declarations.rs
@@ -32,7 +32,7 @@ pub enum FunctionBodyDeclaration<'a> {
 #[derive(Debug, Node)]
 pub struct FunctionBodyDeclarationWithoutPort<'a> {
     pub nodes: (
-        FunctionDataTypeOrImplicit<'a>,
+        Option<FunctionDataTypeOrImplicit<'a>>,
         Option<InterfaceIdentifierOrClassScope<'a>>,
         FunctionIdentifier<'a>,
         Symbol<'a>,
@@ -46,7 +46,7 @@ pub struct FunctionBodyDeclarationWithoutPort<'a> {
 #[derive(Debug, Node)]
 pub struct FunctionBodyDeclarationWithPort<'a> {
     pub nodes: (
-        FunctionDataTypeOrImplicit<'a>,
+        Option<FunctionDataTypeOrImplicit<'a>>,
         Option<InterfaceIdentifierOrClassScope<'a>>,
         FunctionIdentifier<'a>,
         Paren<'a, Option<TfPortList<'a>>>,
@@ -187,9 +187,9 @@ pub fn function_body_declaration(s: Span) -> IResult<Span, FunctionBodyDeclarati
     ))(s)
 }
 
-#[parser]
+#[parser(Ambiguous)]
 pub fn function_body_declaration_without_port(s: Span) -> IResult<Span, FunctionBodyDeclaration> {
-    let (s, a) = function_data_type_or_implicit(s)?;
+    let (s, a) = ambiguous_opt(function_data_type_or_implicit)(s)?;
     let (s, b) = opt(interface_identifier_or_class_scope)(s)?;
     let (s, c) = function_identifier(s)?;
     let (s, d) = symbol(";")(s)?;
@@ -205,9 +205,9 @@ pub fn function_body_declaration_without_port(s: Span) -> IResult<Span, Function
     ))
 }
 
-#[parser]
+#[parser(Ambiguous)]
 pub fn function_body_declaration_with_port(s: Span) -> IResult<Span, FunctionBodyDeclaration> {
-    let (s, a) = function_data_type_or_implicit(s)?;
+    let (s, a) = ambiguous_opt(function_data_type_or_implicit)(s)?;
     let (s, b) = opt(interface_identifier_or_class_scope)(s)?;
     let (s, c) = function_identifier(s)?;
     let (s, d) = paren(opt(tf_port_list))(s)?;
diff --git a/src/parser/declarations/let_declarations.rs b/src/parser/declarations/let_declarations.rs
index f16703e..b36f3bb 100644
--- a/src/parser/declarations/let_declarations.rs
+++ b/src/parser/declarations/let_declarations.rs
@@ -34,7 +34,7 @@ pub struct LetPortList<'a> {
 pub struct LetPortItem<'a> {
     pub nodes: (
         Vec<AttributeInstance<'a>>,
-        LetFormalType<'a>,
+        Option<LetFormalType<'a>>,
         FormalPortIdentifier<'a>,
         Vec<VariableDimension<'a>>,
         Option<(Symbol<'a>, Expression<'a>)>,
@@ -124,10 +124,10 @@ pub fn let_port_list(s: Span) -> IResult<Span, LetPortList> {
     Ok((s, LetPortList { nodes: (a,) }))
 }
 
-#[parser]
+#[parser(Ambiguous)]
 pub fn let_port_item(s: Span) -> IResult<Span, LetPortItem> {
     let (s, a) = many0(attribute_instance)(s)?;
-    let (s, b) = let_formal_type(s)?;
+    let (s, b) = ambiguous_opt(let_formal_type)(s)?;
     let (s, c) = formal_port_identifier(s)?;
     let (s, d) = many0(variable_dimension)(s)?;
     let (s, e) = opt(pair(symbol("="), expression))(s)?;
diff --git a/src/parser/declarations/module_parameter_declarations.rs b/src/parser/declarations/module_parameter_declarations.rs
index a48d9ab..79464cc 100644
--- a/src/parser/declarations/module_parameter_declarations.rs
+++ b/src/parser/declarations/module_parameter_declarations.rs
@@ -16,7 +16,7 @@ pub enum LocalParameterDeclaration<'a> {
 pub struct LocalParameterDeclarationParam<'a> {
     pub nodes: (
         Symbol<'a>,
-        DataTypeOrImplicit<'a>,
+        Option<DataTypeOrImplicit<'a>>,
         ListOfParamAssignments<'a>,
     ),
 }
@@ -36,7 +36,7 @@ pub enum ParameterDeclaration<'a> {
 pub struct ParameterDeclarationParam<'a> {
     pub nodes: (
         Symbol<'a>,
-        DataTypeOrImplicit<'a>,
+        Option<DataTypeOrImplicit<'a>>,
         ListOfParamAssignments<'a>,
     ),
 }
@@ -66,10 +66,10 @@ pub fn local_parameter_declaration(s: Span) -> IResult<Span, LocalParameterDecla
     ))(s)
 }
 
-#[parser]
+#[parser(Ambiguous)]
 pub fn local_parameter_declaration_param(s: Span) -> IResult<Span, LocalParameterDeclaration> {
     let (s, a) = symbol("localparam")(s)?;
-    let (s, b) = data_type_or_implicit(s)?;
+    let (s, b) = ambiguous_opt(data_type_or_implicit)(s)?;
     let (s, c) = list_of_param_assignments(s)?;
     Ok((
         s,
@@ -93,10 +93,10 @@ pub fn parameter_declaration(s: Span) -> IResult<Span, ParameterDeclaration> {
     alt((parameter_declaration_param, parameter_declaration_type))(s)
 }
 
-#[parser]
+#[parser(Ambiguous)]
 pub fn parameter_declaration_param(s: Span) -> IResult<Span, ParameterDeclaration> {
     let (s, a) = symbol("parameter")(s)?;
-    let (s, b) = data_type_or_implicit(s)?;
+    let (s, b) = ambiguous_opt(data_type_or_implicit)(s)?;
     let (s, c) = list_of_param_assignments(s)?;
     Ok((
         s,
diff --git a/src/parser/declarations/net_and_variable_types.rs b/src/parser/declarations/net_and_variable_types.rs
index 09f26df..53cf946 100644
--- a/src/parser/declarations/net_and_variable_types.rs
+++ b/src/parser/declarations/net_and_variable_types.rs
@@ -533,12 +533,12 @@ pub fn net_type(s: Span) -> IResult<Span, NetType> {
     alt((
         map(symbol("supply0"), |x| NetType::Supply0(x)),
         map(symbol("supply1"), |x| NetType::Supply1(x)),
-        map(symbol("tri"), |x| NetType::Tri(x)),
         map(symbol("triand"), |x| NetType::Triand(x)),
         map(symbol("trior"), |x| NetType::Trior(x)),
         map(symbol("trireg"), |x| NetType::Trireg(x)),
         map(symbol("tri0"), |x| NetType::Tri0(x)),
         map(symbol("tri1"), |x| NetType::Tri1(x)),
+        map(symbol("tri"), |x| NetType::Tri(x)),
         map(symbol("uwire"), |x| NetType::Uwire(x)),
         map(symbol("wire"), |x| NetType::Wire(x)),
         map(symbol("wand"), |x| NetType::Wand(x)),
diff --git a/src/parser/declarations/port_declarations.rs b/src/parser/declarations/port_declarations.rs
index 525e0bb..c842863 100644
--- a/src/parser/declarations/port_declarations.rs
+++ b/src/parser/declarations/port_declarations.rs
@@ -9,7 +9,11 @@ use nom::IResult;
 
 #[derive(Debug, Node)]
 pub struct InoutDeclaration<'a> {
-    pub nodes: (Symbol<'a>, NetPortType<'a>, ListOfPortIdentifiers<'a>),
+    pub nodes: (
+        Symbol<'a>,
+        Option<NetPortType<'a>>,
+        ListOfPortIdentifiers<'a>,
+    ),
 }
 
 #[derive(Debug, Node)]
@@ -20,7 +24,11 @@ pub enum InputDeclaration<'a> {
 
 #[derive(Debug, Node)]
 pub struct InputDeclarationNet<'a> {
-    pub nodes: (Symbol<'a>, NetPortType<'a>, ListOfPortIdentifiers<'a>),
+    pub nodes: (
+        Symbol<'a>,
+        Option<NetPortType<'a>>,
+        ListOfPortIdentifiers<'a>,
+    ),
 }
 
 #[derive(Debug, Node)]
@@ -40,7 +48,11 @@ pub enum OutputDeclaration<'a> {
 
 #[derive(Debug, Node)]
 pub struct OutputDeclarationNet<'a> {
-    pub nodes: (Symbol<'a>, NetPortType<'a>, ListOfPortIdentifiers<'a>),
+    pub nodes: (
+        Symbol<'a>,
+        Option<NetPortType<'a>>,
+        ListOfPortIdentifiers<'a>,
+    ),
 }
 
 #[derive(Debug, Node)]
@@ -72,10 +84,10 @@ pub struct RefDeclaration<'a> {
 
 // -----------------------------------------------------------------------------
 
-#[parser]
+#[parser(Ambiguous)]
 pub fn inout_declaration(s: Span) -> IResult<Span, InoutDeclaration> {
     let (s, a) = symbol("inout")(s)?;
-    let (s, b) = net_port_type(s)?;
+    let (s, b) = ambiguous_opt(net_port_type)(s)?;
     let (s, c) = list_of_port_identifiers(s)?;
     Ok((s, InoutDeclaration { nodes: (a, b, c) }))
 }
@@ -85,10 +97,10 @@ pub fn input_declaration(s: Span) -> IResult<Span, InputDeclaration> {
     alt((input_declaration_net, input_declaration_variable))(s)
 }
 
-#[parser]
+#[parser(Ambiguous)]
 pub fn input_declaration_net(s: Span) -> IResult<Span, InputDeclaration> {
     let (s, a) = symbol("input")(s)?;
-    let (s, b) = net_port_type(s)?;
+    let (s, b) = ambiguous_opt(net_port_type)(s)?;
     let (s, c) = list_of_port_identifiers(s)?;
     Ok((
         s,
@@ -112,10 +124,10 @@ pub fn output_declaration(s: Span) -> IResult<Span, OutputDeclaration> {
     alt((output_declaration_net, output_declaration_variable))(s)
 }
 
-#[parser]
+#[parser(Ambiguous)]
 pub fn output_declaration_net(s: Span) -> IResult<Span, OutputDeclaration> {
     let (s, a) = symbol("output")(s)?;
-    let (s, b) = net_port_type(s)?;
+    let (s, b) = ambiguous_opt(net_port_type)(s)?;
     let (s, c) = list_of_port_identifiers(s)?;
     Ok((
         s,
@@ -149,3 +161,18 @@ pub fn ref_declaration(s: Span) -> IResult<Span, RefDeclaration> {
     let (s, c) = list_of_variable_identifiers(s)?;
     Ok((s, RefDeclaration { nodes: (a, b, c) }))
 }
+
+// -----------------------------------------------------------------------------
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_inout_declaration() {
+        parser_test!(inout_declaration, "inout a", Ok((_, _)));
+        parser_test!(inout_declaration, "inout [7:0] a", Ok((_, _)));
+        parser_test!(inout_declaration, "inout signed [7:0] a", Ok((_, _)));
+        parser_test!(inout_declaration, "inout var a", Ok((_, _)));
+    }
+}
diff --git a/src/parser/declarations/task_declarations.rs b/src/parser/declarations/task_declarations.rs
index 7841cbe..ff774f5 100644
--- a/src/parser/declarations/task_declarations.rs
+++ b/src/parser/declarations/task_declarations.rs
@@ -63,7 +63,7 @@ pub struct TfPortItem<'a> {
         Vec<AttributeInstance<'a>>,
         Option<TfPortDirection<'a>>,
         Option<Var<'a>>,
-        DataTypeOrImplicit<'a>,
+        Option<DataTypeOrImplicit<'a>>,
         Option<(
             PortIdentifier<'a>,
             Vec<VariableDimension<'a>>,
@@ -84,7 +84,7 @@ pub struct TfPortDeclaration<'a> {
         Vec<AttributeInstance<'a>>,
         TfPortDirection<'a>,
         Option<Var<'a>>,
-        DataTypeOrImplicit<'a>,
+        Option<DataTypeOrImplicit<'a>>,
         ListOfTfVariableIdentifiers<'a>,
         Symbol<'a>,
     ),
@@ -170,12 +170,12 @@ pub fn tf_port_list(s: Span) -> IResult<Span, TfPortList> {
     Ok((s, TfPortList { nodes: (a,) }))
 }
 
-#[parser]
+#[parser(Ambiguous)]
 pub fn tf_port_item(s: Span) -> IResult<Span, TfPortItem> {
     let (s, a) = many0(attribute_instance)(s)?;
     let (s, b) = opt(tf_port_direction)(s)?;
     let (s, c) = opt(var)(s)?;
-    let (s, d) = data_type_or_implicit(s)?;
+    let (s, d) = ambiguous_opt(data_type_or_implicit)(s)?;
     let (s, e) = opt(triple(
         port_identifier,
         many0(variable_dimension),
@@ -199,12 +199,12 @@ pub fn tf_port_direction(s: Span) -> IResult<Span, TfPortDirection> {
     ))(s)
 }
 
-#[parser]
+#[parser(Ambiguous)]
 pub fn tf_port_declaration(s: Span) -> IResult<Span, TfPortDeclaration> {
     let (s, a) = many0(attribute_instance)(s)?;
     let (s, b) = tf_port_direction(s)?;
     let (s, c) = opt(var)(s)?;
-    let (s, d) = data_type_or_implicit(s)?;
+    let (s, d) = ambiguous_opt(data_type_or_implicit)(s)?;
     let (s, e) = list_of_tf_variable_identifiers(s)?;
     let (s, f) = symbol(";")(s)?;
     Ok((
diff --git a/src/parser/declarations/type_declarations.rs b/src/parser/declarations/type_declarations.rs
index 104b069..4607585 100644
--- a/src/parser/declarations/type_declarations.rs
+++ b/src/parser/declarations/type_declarations.rs
@@ -22,7 +22,7 @@ pub struct DataDeclarationVariable<'a> {
         Option<Const<'a>>,
         Option<Var<'a>>,
         Option<Lifetime<'a>>,
-        DataTypeOrImplicit<'a>,
+        Option<DataTypeOrImplicit<'a>>,
         ListOfVariableDeclAssignments<'a>,
         Symbol<'a>,
     ),
@@ -96,7 +96,7 @@ pub struct NetDeclarationNetType<'a> {
         NetType<'a>,
         Option<Strength<'a>>,
         Option<VectorScalar<'a>>,
-        DataTypeOrImplicit<'a>,
+        Option<DataTypeOrImplicit<'a>>,
         Option<Delay3<'a>>,
         ListOfNetDeclAssignments<'a>,
         Symbol<'a>,
@@ -242,12 +242,12 @@ pub fn data_declaration(s: Span) -> IResult<Span, DataDeclaration> {
     ))(s)
 }
 
-#[parser]
+#[parser(Ambiguous)]
 pub fn data_declaration_variable(s: Span) -> IResult<Span, DataDeclaration> {
     let (s, a) = opt(r#const)(s)?;
     let (s, b) = opt(var)(s)?;
     let (s, c) = opt(lifetime)(s)?;
-    let (s, d) = data_type_or_implicit(s)?;
+    let (s, d) = ambiguous_opt(data_type_or_implicit)(s)?;
     let (s, e) = list_of_variable_decl_assignments(s)?;
     let (s, f) = symbol(";")(s)?;
     Ok((
@@ -340,18 +340,18 @@ pub fn genvar_declaration(s: Span) -> IResult<Span, GenvarDeclaration> {
 #[parser]
 pub fn net_declaration(s: Span) -> IResult<Span, NetDeclaration> {
     alt((
+        net_declaration_interconnect,
         net_declaration_net_type,
         net_declaration_net_type_identifier,
-        net_declaration_interconnect,
     ))(s)
 }
 
-#[parser]
+#[parser(Ambiguous)]
 pub fn net_declaration_net_type(s: Span) -> IResult<Span, NetDeclaration> {
     let (s, a) = net_type(s)?;
     let (s, b) = opt(strength)(s)?;
     let (s, c) = opt(vector_scalar)(s)?;
-    let (s, d) = data_type_or_implicit(s)?;
+    let (s, d) = ambiguous_opt(data_type_or_implicit)(s)?;
     let (s, e) = opt(delay3)(s)?;
     let (s, f) = list_of_net_decl_assignments(s)?;
     let (s, g) = symbol(";")(s)?;
@@ -531,3 +531,75 @@ pub fn lifetime(s: Span) -> IResult<Span, Lifetime> {
         map(symbol("automatic"), |x| Lifetime::Automatic(x)),
     ))(s)
 }
+
+// -----------------------------------------------------------------------------
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_net_type_declaration() {
+        parser_test!(
+            net_type_declaration,
+            "nettype T wT;",
+            Ok((_, NetTypeDeclaration::DataType(_)))
+        );
+        parser_test!(
+            net_type_declaration,
+            "nettype T wTsum with Tsum;",
+            Ok((_, NetTypeDeclaration::DataType(_)))
+        );
+        parser_test!(
+            net_type_declaration,
+            "nettype MyBaseT::T narrowTsum with MyBaseT::Tsum;",
+            Ok((_, NetTypeDeclaration::DataType(_)))
+        );
+    }
+
+    #[test]
+    fn test_net_declaration() {
+        parser_test!(
+            net_declaration,
+            "trireg (large) logic #(0,0,0) cap1;",
+            Ok((_, NetDeclaration::NetType(_)))
+        );
+        parser_test!(
+            net_declaration,
+            "wire addressT w1;",
+            Ok((_, NetDeclaration::NetType(_)))
+        );
+        parser_test!(
+            net_declaration,
+            "wire struct packed { logic ecc; logic [7:0] data; } memsig;",
+            Ok((_, NetDeclaration::NetType(_)))
+        );
+        parser_test!(
+            net_declaration,
+            "wire w;",
+            Ok((_, NetDeclaration::NetType(_)))
+        );
+        parser_test!(
+            net_declaration,
+            "wire [15:0] w;",
+            Ok((_, NetDeclaration::NetType(_)))
+        );
+        parser_test!(
+            net_declaration,
+            "interconnect w1;",
+            Ok((_, NetDeclaration::Interconnect(_)))
+        );
+        parser_test!(
+            net_declaration,
+            "interconnect [3:0] w2;",
+            Ok((_, NetDeclaration::Interconnect(_)))
+        );
+        parser_test!(
+            net_declaration,
+            "interconnect [3:0] w3 [1:0];",
+            Ok((_, NetDeclaration::Interconnect(_)))
+        );
+        parser_test!(net_declaration, "interconnect logic [3:0] w4;", Err(_));
+        parser_test!(net_declaration, "interconnect #(1,2,3) w5;", Err(_));
+    }
+}
diff --git a/src/parser/expressions/expression_leftside_values.rs b/src/parser/expressions/expression_leftside_values.rs
index 5c9859a..9d86e0a 100644
--- a/src/parser/expressions/expression_leftside_values.rs
+++ b/src/parser/expressions/expression_leftside_values.rs
@@ -156,41 +156,6 @@ pub fn nonrange_variable_lvalue(s: Span) -> IResult<Span, NonrangeVariableLvalue
     Ok((s, NonrangeVariableLvalue { nodes: (a, b, c) }))
 }
 
-#[derive(Debug, Node)]
-pub enum Expr<'a> {
-    Expr(Box<ExprExpr<'a>>),
-    Term(ExprTerm<'a>),
-}
-
-#[derive(Debug, Node)]
-pub struct ExprExpr<'a> {
-    pub nodes: (Expr<'a>, Symbol<'a>, Expr<'a>),
-}
-
-#[derive(Debug, Node)]
-pub struct ExprTerm<'a> {
-    pub nodes: (Identifier<'a>,),
-}
-
-#[parser]
-pub fn expr(s: Span) -> IResult<Span, Expr> {
-    alt((expr_expr, expr_term))(s)
-}
-
-#[parser(MaybeRecursive)]
-pub fn expr_expr(s: Span) -> IResult<Span, Expr> {
-    let (s, a) = expr(s)?;
-    let (s, b) = symbol("+")(s)?;
-    let (s, c) = expr(s)?;
-    Ok((s, Expr::Expr(Box::new(ExprExpr { nodes: (a, b, c) }))))
-}
-
-#[parser]
-pub fn expr_term(s: Span) -> IResult<Span, Expr> {
-    let (s, a) = identifier(s)?;
-    Ok((s, Expr::Term(ExprTerm { nodes: (a,) })))
-}
-
 // -----------------------------------------------------------------------------
 
 #[cfg(test)]
@@ -219,10 +184,4 @@ mod tests {
         //parser_test!(nonrange_variable_lvalue, "A[][2][3]", Ok((_, _)));
         //parser_test!(nonrange_variable_lvalue, "A[][]", Ok((_, _)));
     }
-
-    #[test]
-    fn test_expr() {
-        parser_test!(expr, "A", Ok((_, _)));
-        parser_test!(expr, "A+B", Ok((_, _)));
-    }
 }
diff --git a/src/parser/utils.rs b/src/parser/utils.rs
index dd9bc2e..e959ee2 100644
--- a/src/parser/utils.rs
+++ b/src/parser/utils.rs
@@ -188,6 +188,13 @@ where
     }
 }
 
+pub fn none<'a, O, F>(_f: F) -> impl Fn(Span<'a>) -> IResult<Span<'a>, Option<O>>
+where
+    F: Fn(Span<'a>) -> IResult<Span<'a>, O>,
+{
+    move |s: Span<'a>| Ok((s, None))
+}
+
 // -----------------------------------------------------------------------------
 
 #[parser]
diff --git a/sv-parser-derive/Cargo.toml b/sv-parser-derive/Cargo.toml
index f0d1460..b4d8b85 100644
--- a/sv-parser-derive/Cargo.toml
+++ b/sv-parser-derive/Cargo.toml
@@ -10,5 +10,5 @@ edition = "2018"
 proc-macro = true
 
 [dependencies]
-syn = { version = "0.15.18", features = ["full", "fold", "parsing"] }
+syn = { version = "0.15.18", features = ["full", "fold", "parsing", "extra-traits"] }
 quote = "0.6.9"
diff --git a/sv-parser-derive/src/lib.rs b/sv-parser-derive/src/lib.rs
index db25bdc..9bd42cc 100644
--- a/sv-parser-derive/src/lib.rs
+++ b/sv-parser-derive/src/lib.rs
@@ -4,6 +4,7 @@ extern crate proc_macro;
 
 use crate::proc_macro::TokenStream;
 use quote::quote;
+use std::str::FromStr;
 use syn::Data::{Enum, Struct};
 use syn::{
     self, parse_macro_input, AttributeArgs, DeriveInput, Expr, ItemFn, Meta, NestedMeta, Stmt,
@@ -121,20 +122,75 @@ pub fn parser(attr: TokenStream, item: TokenStream) -> TokenStream {
 }
 
 fn impl_parser(attr: &AttributeArgs, item: &ItemFn) -> TokenStream {
-    let (maybe_recursive,) = impl_parser_attribute(attr);
+    let (maybe_recursive, ambiguous) = impl_parser_attribute(attr);
+
+    let trace = impl_parser_trace(&item);
+    let trace = parse_macro_input!(trace as Stmt);
+
+    let check_recursive_flag = impl_parser_check_recursive_flag(&item);
+    let check_recursive_flag = parse_macro_input!(check_recursive_flag as Stmt);
+
+    let set_recursive_flag = impl_parser_set_recursive_flag(&item);
+    let set_recursive_flag = parse_macro_input!(set_recursive_flag as Stmt);
+
+    let body = if ambiguous {
+        impl_parser_body_ambiguous(&item)
+    } else {
+        impl_parser_body(&item)
+    };
+    let body = parse_macro_input!(body as Stmt);
+
+    let clear_recursive_flags = impl_parser_clear_recursive_flags(&item);
+    let clear_recursive_flags = parse_macro_input!(clear_recursive_flags as Expr);
+    let clear_recursive_flags = Stmt::Expr(clear_recursive_flags);
 
-    let ident = &item.ident;
     let mut item = item.clone();
 
-    let tracer = quote! {
+    item.block.stmts.clear();
+    item.block.stmts.push(trace);
+    if maybe_recursive {
+        item.block.stmts.push(check_recursive_flag);
+        item.block.stmts.push(set_recursive_flag);
+    }
+    item.block.stmts.push(body);
+    item.block.stmts.push(clear_recursive_flags);
+
+    let gen = quote! {
+        #item
+    };
+    gen.into()
+}
+
+fn impl_parser_attribute(attr: &AttributeArgs) -> (bool, bool) {
+    let mut maybe_recursive = false;
+    let mut ambiguous = false;
+
+    for a in attr {
+        match a {
+            NestedMeta::Meta(Meta::Word(x)) if x == "MaybeRecursive" => maybe_recursive = true,
+            NestedMeta::Meta(Meta::Word(x)) if x == "Ambiguous" => ambiguous = true,
+            _ => panic!(),
+        }
+    }
+
+    (maybe_recursive, ambiguous)
+}
+
+fn impl_parser_trace(item: &ItemFn) -> TokenStream {
+    let ident = &item.ident;
+
+    let gen = quote! {
         if cfg!(feature = "trace") {
             println!("{:<64} : {:<4},{:>032x} : {}", stringify!(#ident), s.offset, s.extra[0], s.fragment);
         }
     };
-    let tracer: TokenStream = tracer.into();
-    let tracer = parse_macro_input!(tracer as Stmt);
+    gen.into()
+}
 
-    let checker = quote! {
+fn impl_parser_check_recursive_flag(item: &ItemFn) -> TokenStream {
+    let ident = &item.ident;
+
+    let gen = quote! {
         if thread_context::PARSER_INDEX.with(|p| {
             if let Some(i) = p.borrow_mut().get(stringify!(#ident)) {
                 return check_recursive_flag(s, i);
@@ -148,10 +204,13 @@ fn impl_parser(attr: &AttributeArgs, item: &ItemFn) -> TokenStream {
             return Err(nom::Err::Error(nom::error::make_error(s, nom::error::ErrorKind::Fix)));
         }
     };
-    let checker: TokenStream = checker.into();
-    let checker = parse_macro_input!(checker as Stmt);
+    gen.into()
+}
 
-    let before = quote! {
+fn impl_parser_set_recursive_flag(item: &ItemFn) -> TokenStream {
+    let ident = &item.ident;
+
+    let gen = quote! {
         let s = thread_context::PARSER_INDEX.with(|p| {
             if let Some(i) = p.borrow_mut().get(stringify!(#ident)) {
                 set_recursive_flag(s, i, true)
@@ -163,53 +222,80 @@ fn impl_parser(attr: &AttributeArgs, item: &ItemFn) -> TokenStream {
             }
         });
     };
-    let before: TokenStream = before.into();
-    let before = parse_macro_input!(before as Stmt);
+    gen.into()
+}
 
-    let mut body = quote! {};
+fn impl_parser_body(item: &ItemFn) -> TokenStream {
+    let mut gen = quote! {};
     for s in &item.block.stmts {
-        body = quote! {
-            #body
+        gen = quote! {
+            #gen
             #s
         };
     }
-    let body = quote! {
-        let (s, ret) = { #body }?;
-    };
-    let body: TokenStream = body.into();
-    let body = parse_macro_input!(body as Stmt);
-
-    let after = quote! {
-        Ok((clear_recursive_flags(s), ret))
-    };
-    let after: TokenStream = after.into();
-    let after = parse_macro_input!(after as Expr);
-    let after = Stmt::Expr(after);
-
-    item.block.stmts.clear();
-    item.block.stmts.push(tracer);
-    if maybe_recursive {
-        item.block.stmts.push(checker);
-        item.block.stmts.push(before);
-    }
-    item.block.stmts.push(body);
-    item.block.stmts.push(after);
-
     let gen = quote! {
-        #item
+        let (s, ret) = { #gen }?;
     };
     gen.into()
 }
 
-fn impl_parser_attribute(attr: &AttributeArgs) -> (bool,) {
-    let mut maybe_recursive = false;
+fn impl_parser_body_ambiguous(item: &ItemFn) -> TokenStream {
+    let mut token = quote! {};
+    for s in &item.block.stmts {
+        token = quote! {
+            #token
+            #s
+        };
+    }
+    let mut token = token.to_string();
 
-    for a in attr {
-        match a {
-            NestedMeta::Meta(Meta::Word(x)) if x == "MaybeRecursive" => maybe_recursive = true,
-            _ => panic!(),
+    let ambiguous_cnt: Vec<&str> = token.matches("ambiguous").collect();
+    let ambiguous_cnt = ambiguous_cnt.len();
+
+    let mut replace_parsers = Vec::new();
+    for i in 0..ambiguous_cnt {
+        let pos = token.find("ambiguous").unwrap();
+        let (head, rest) = token.split_at(pos);
+        if rest.starts_with("ambiguous_opt") {
+            let rest = rest.replacen("ambiguous_opt", &format!("amb_temporary{}", i), 1);
+            token = format!("{}{}", head, rest);
+            replace_parsers.push(("opt", "none"));
         }
     }
 
-    (maybe_recursive,)
+    let mut gen = quote! {};
+    for i in 0..2_u32.pow(ambiguous_cnt as u32) {
+        let mut token = token.clone();
+        for j in 0..ambiguous_cnt {
+            let (p0, p1) = replace_parsers[j];
+            let repl = if ((i >> j) & 1) == 0 { p0 } else { p1 };
+            token = token.replace(&format!("amb_temporary{}", j), repl);
+        }
+        let token = format!("{{ {} }}", token);
+        let token = TokenStream::from_str(&token).unwrap();
+        let token = parse_macro_input!(token as Stmt);
+        gen = quote! {
+            #gen
+            |s| #token,
+        };
+    }
+
+    let gen = quote! {
+        alt((
+            #gen
+        ))(s)
+    };
+
+    let gen = quote! {
+        let (s, ret) = { #gen }?;
+    };
+
+    gen.into()
+}
+
+fn impl_parser_clear_recursive_flags(_item: &ItemFn) -> TokenStream {
+    let gen = quote! {
+        Ok((clear_recursive_flags(s), ret))
+    };
+    gen.into()
 }