Function parse_qualifier_param 

fn parse_qualifier_param(
    cx: &mut ParseCtxt<'_>,
) -> ParseResult<(RefineParam, bool)>

⟨qualifier_param⟩ := #? ⟨refine_param⟩

#a: int rather than fixpoint’s a#: int because rustc lexes the enclosing attribute first and rejects a# as a reserved prefix.