How can I change the arguments of a generic function in MCL?

[ueda@shpcsl.sharp.co.jp (UEDA masaya)]

How can I change the arguments of a generic function in MCL environment?

   Date: Tue, 07 Jul 92 19:41:48 EDT
   From: moon (David A. Moon)

   You assumed when you read something about the language that the
   development environment would not let you change the arguments of a
   generic function.  That's jumping to a conclusion and I personally
   would consider any development environment that didn't let you do that
   to be pretty deficient.

I came across the message "Error: Incompatible lambda list" when I
tried to define an :after method and add a new keyword argument there. 
Original lambda list has not any keyword argument.

Any suggestion would be appreciated.

Thank you.

Masaya UEDA