enum ItemA root or block child produced by one parser category.
File and block parses contain declaration/statement items; category-specific entry points wrap their single expression, pattern, or type root here so every ParseOutcome has one uniform item array.
Cases
case decl_item(Decl)
Wraps a declaration node.
case stmt_item(Stmt)
Wraps a statement node.
case expr_item(Expr)
A bare expression root (the expr category parse).
case pattern_item(Pattern)
A bare pattern root (the pattern category parse).
case type_item(TypeAnnotation)
A bare type-annotation root (the type category parse).