Keys clean of all signatures except those made by others I trust

[FuzzyDrawrings]

Title says it all.

Say I import Bob's key with "--recv-key" from some keyserver. Bob's public key has been signed by a lot of non-serious User ID's and spam. However Bob's key may have been signed by Alice (whose public-key I have in my keyring).

I would like to clean the key of the spam signatures while preserving any signatures made by Alice (or anyone else I have trusted on my keyring). Does there exist a command/option to accomplish this in gpg2?
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