-- abs_vars = tvars only: return True if e is trivial,
-- but False for anything bigger
-- abs_vars = [x] (an Id): return True for trivial, or an application (f x)
-- abs_vars = tvars only: return True if e is trivial,
-- but False for anything bigger
-- abs_vars = [x] (an Id): return True for trivial, or an application (f x)