unprotect conjugate?



I want to teach maxima how to properly treat the derivative of the conjugate 
function,
with the end result that I will get the behavior:

diff(  conjugate( f(x) ), x );   ==>  conjugate(   'diff( f( x ), x,  1)   )

Is there a way to "unprotect" system functions like conjugate  so this rule 
can be  implemented
for any function f(x)?

TIA
ted woollett