How to reuse pytest fixtures chain without fixture body duplicates?

I have long chain of fixtures with different scopes. I want cache it and redefine first fixtures of chain sometimes. Code below doesn't work cause b(session-scope fixture) depends on a(function scope). I know that I can copy b and c bodies to test_1.py file and change scopes. But it's WET. Is there some way to do that? May be pytest plugins.

conftest.py:

import pytest

@pytest.fixture('session')
def a():
    return 'a'

@pytest.fixture('session')
def b(a):
    return a + 'b'

@pytest.fixture('module')
def c(b):
    return b + ' c'

test_1.py:

import pytest

@fixture('function')
def a():
    return 'a1'

def test_c(c):
    assert c == 'a1 b c'