Logic, how does it work?

logic.py

def parse(expr):
  lst = []

  for line in expr.split('\n'):
    s = line.split(' -> ')
    if len(s) == 1:
      lst.append(('def', line))
    else:
      lst.append(('imply', s[0], s[1]))

  return lst

def leval(expr):
  sym = {}

  for p in expr:
    if p[0] == 'def':
      sym[p[1]] = True
    elif p[0] == 'imply':
      if sym.setdefault(p[1], False) is True:
        sym[p[2]] = True
      elif not p[2] in sym:
        sym[p[2]] = False

  return sym

def main():
  p = parse("""F
F -> G
H -> F
Y -> L
H""")
  
  assert p == [('def', 'F'),
               ('imply', 'F', 'G'),
               ('imply', 'H', 'F'),
               ('imply', 'Y', 'L'),
               ('def', 'H')]

  print leval(p)

main()