CL-USER> (make-instance 'c2mop:funcallable-standard-object)
#<SB-MOP:FUNCALLABLE-STANDARD-OBJECT {1010B71EEB}>

CL-USER> (c2mop:set-funcallable-instance-function * (lambda (x) (* x x)))
#<FUNCTION (LAMBDA (X)) {53880E6B}>

CL-USER> (funcall ** 42)
1764