set-funcallable-instance-function (fin new-value)

The funcallable-instance argument is a funcallable instance (it must have been returned by allocate-instance (funcallable-standard-class)).
The function argument is a function.

The value returned by this function is unspecified.

This function is called to set or to change the function of a funcallable instance. After set-funcallable-instance-function is called, any subsequent calls to funcallable-instance will run the new function.