isFunLhs :: LHsExpr RdrName
-> P (Maybe (Located RdrName, Bool, [LHsExpr RdrName]))
-- Just (fun, is_infix, arg_pats) if e is a function LHS
isFunLhs :: LHsExpr RdrName
-> P (Maybe (Located RdrName, Bool, [LHsExpr RdrName]))
-- Just (fun, is_infix, arg_pats) if e is a function LHS