PGP: pgp -ke

[Chuck Lever]

   so, like, what's to stop me from writing a program (based on pgp source
   code) which can delete user IDs from my own keys after other folks have
   signed them?  in fact, how *can* i change the user ID on a key after it
   has been signed?

   the pgp docs are unclear on how this works.  can someone help me to
   understand what it means exactly when a key is signed?  what parts of
   the key are certified by the signature?