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
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
def __init__(self, parent, left_idem, strands, right_idem=None):
"""Be sure to use MinusStrands to convert strands."""
if not isinstance(strands, MinusStrands):
strands = MinusStrands(parent.pmc, strands)
StrandDiagram.__init__(self, parent, left_idem, strands, right_idem)
