use __contains__, index or split instead of str.find
str.find return -1 when the substring is not found, -1 evaluate
to True and is a valid index, which can lead to bugs.
Using alternatives when possible makes the code clearer and less
prone to bugs. (and __contains__ is faster in microbenchmarks)
# should print nothing
# should print ? b/x
? b/x
# should print A b/x
A b/x
# should forget b/x
forgetting b/x
# should print nothing