"Usually not allowed": rather the behavior is undefined, it is not a conforming program. (and in practice the error is "Lock on package COMMON-LISP violated wh

"Usually not allowed": rather the behavior is undefined, it is not a conforming program. (and in practice the error is "Lock on package COMMON-LISP violated when binding NOT as a local function", you can lock your own packages too if you want). Also, you generally mind your own symbols, you don't rebind symbols from other packages (hygienic macros are more useful when you only have one namespace).