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else:

If a letter in mapA[letter] exists in mapB[letter], add

that letter to intersectedMapping[letter].

for mappedLetter in mapA[letter]:

if mappedLetter in mapB[letter]:

intersectedMapping[letter].append(mappedLetter)

return intersectedMapping

def removeSolvedLettersFromMapping(letterMapping):

Cipher letters in the mapping that map to only one letter are

"solved" and can be removed from the other letters.

For example, if 'A' maps to potential letters ['M', 'N'], and 'B'

maps to ['N'], then we know that 'B' must map to 'N', so we can

remove 'N' from the list of what 'A' could map to. So 'A' then maps

to ['M']. Note that now that 'A' maps to only one letter, we can

remove 'M' from the list of letters for every other

letter. (This is why there is a loop that keeps reducing the map.)

letterMapping = copy.deepcopy(letterMapping)

loopAgain = True

while loopAgain:

First assume that we will not loop again:

loopAgain = False

solvedLetters will be a list of uppercase letters that have one

and only one possible mapping in letterMapping

solvedLetters = []

for cipherletter in LETTERS:

if len(letterMapping[cipherletter]) == 1:

solvedLetters.append(letterMapping[cipherletter][0])

If a letter is solved, than it cannot possibly be a potential

decryption letter for a different ciphertext letter, so we

should remove it from those other lists.

for cipherletter in LETTERS:

for s in solvedLetters:

if len(letterMapping[cipherletter]) != 1 and s in
letterMapping[cipherletter]:

letterMapping[cipherletter].remove(s)