def _cross(sd, start1, end1, start2, end2):
"""Cross the two strands (start1, end1) and (start2, end2) of the
strand diagram on the given side of start_gen.
"""
strand_lst = list(sd.strands)
strand_lst.remove((start1, end1))
strand_lst.remove((start2, end2))
if start1 != end2:
strand_lst.append((start1, end2))
if start2 != end1:
strand_lst.append((start2, end1))
result_sd = StrandDiagram(sd.parent, sd.getLeftIdem(), strand_lst)
assert sd in result_sd.diff()
return result_sd
