d07_dirty.py

>>> lines = c.split('\n')
>>> S1 = set()
>>> S2 = set()
>>> for line in lines:
...  S1.add(line.split()[0])
...  if '->' in line:
...   for stacked in line.split('->')[-1].split(','):
...    S2.add(stacked.strip())
...
>>> S1 - S2