>> Terry's and Martin' example sentences are transferable to that.
>> However, the actual sentence was
>> "dict.has_key(key) is equivalent to key in d, but deprecated."

> The sentence being changed was perfectly and mathematically correct.

I'm not so sure about that anymore. Reading the post in

http://www.englishforums.com/English/RepetitionSubjectPronoun/cqjlw/post.htm

it now seems to me that the part after the comma is a separate sentence,
and both the subject pronoun and the verb are omitted (as an ellipsis).
So the full sentence would indeed read

"dict.has_key(key) is equivalent to key in d, but it is deprecated."

which then would be ambiguous as discussed.