Splurthian Chemistry 103 submission scoring script

candidates = set(map(str.strip, open("candidates.txt")))
submission = list(map(str.strip, open("submission.txt")))
assert all(element in candidates for element in submission)
assert submission == sorted(submission)
def symbols(element):
    for i, c1 in enumerate(element):
        for c2 in element[i + 1:]:
            yield c1 + c2
symbollist = []
for element in open("submission.txt"):
    element = element.strip()
    for symbol in symbols(element):
        if symbol not in symbollist:
            symbollist.append(symbol)
            break
    else:
        raise "No available valid symbol for " + element
symbollist.reverse()
while symbollist != sorted(symbollist):
    symbollist.pop()
print(len(symbollist))