Story steps should be of the same kind.

[proofit404]

a1s1 can't be coroutine while a1s2 is a function.

A suggested implementation:

steps = ['a1s1','a1s2','b1']
methods = {'a1s1': f, 'a1s2': async f}
kinds = {is_coroutine_function, methods.values()} # set generator
if kinds == {True, False}:
    raise exception (class has mixed methods)