def _factor(sd, a, b):
"""Try to factor off a strand (a, b) (from the left) from sd. Returns
the new strand diagram if successful. Otherwise returns None.
"""
strand_lst = list(sd.strands)
for p, q in [(p, q) for p,q in strand_lst if p == a and q >= b]:
strand_lst.remove((p, q))
if q > b:
strand_lst.append((b, q))
result = StrandDiagram(sd.parent, None, strand_lst,
sd.getRightIdem())
to_mult = StrandDiagram(sd.parent, None, [(a, b)],
result.getLeftIdem())
if to_mult * result == 1 * sd:
return result
else:
return None
