Prime implicant finding uses an unnecessarily slow algorithm

[TheLoneWolfling]

Current prime implicant finding uses an algorithm that works out to be O(|nonzero miniterms|^2) time.

However, it's possible to get prime implicants in O(|nonzero miniterms|) time using a set instead of explicit comparisons:

current = {"0000", "1001", ...}
primImps = set()
while current:
  nxt = set()
  for c in current:
    added = False
    for i, old in enumerate(c):
      for new in "01x":
        if old == new:
          continue
        d = c[:i] + new + c[i+1:]
        if d in current:
          n = "".join(x if x == y else 'x' for x,y in zip(c,d))
          nxt.add(n)
          added = True
    if not added:
      primImps.add(c)
  current = nxt

(Well, technically there's a O(|miniterm|) factor in there - but that's present regardless.)
This (drastically) speeds up larger problems.