; in: DEFMETHOD FLATMAP (OBSERVABLE FUNCTION)
;     (LENGTH (ARIA.CONTROL.RX::INNERS ARIA.CONTROL.RX::SUBSCRIBER))
; 
; note: unable to
;   optimize
; due to type uncertainty:
;   The first argument is a SEQUENCE, not a (SIMPLE-ARRAY * (*)).
; 
; note: unable to
;   optimize
; due to type uncertainty:
;   The first argument is a SEQUENCE, not a VECTOR.