new_contents: import pytest
  from skiski.bfc import B
  
  
  def test_composite_function():
      a = lambda x: x * 5
      b = lambda x: x - 3
      assert B(a).dot(b).dot(5).w() == 10
  
  
  def test_sksk_is_b():
      a = lambda x: x * 5
      b = lambda x: x - 3
      b_comb = B(a).dot(b).dot(5).w()
      sksk = B.to_ski().dot(a).w().dot(b).dot(5).w()
      assert b_comb == sksk
  
subject: add B, F, C Combinator test