let F, F1, F2 be FinSequence of COMPLEX ; :: thesis:
A1: dom multcomplex = [:COMPLEX,COMPLEX:] by FUNCT_2:def 1;
then A2: [:(rng F2),(rng F1):] c= dom multcomplex by ZFMISC_1:96;
[:(rng F1),(rng F2):] c= dom multcomplex by A1, ZFMISC_1:96;
then A3: dom (multcomplex .: (F1,F2)) = (dom F1) /\ (dom F2) by FUNCOP_1:69
.= dom (multcomplex .: (F2,F1)) by A2, FUNCOP_1:69 ;
assume A4: F = multcomplex .: (F1,F2) ; :: thesis:
for z being set st z in dom (multcomplex .: (F2,F1)) holds
F . z = multcomplex . ((F2 . z),(F1 . z))

proof

let z be set ; :: thesis:
assume A5: z in dom (multcomplex .: (F2,F1)) ; :: thesis:
set F1z = F1 . z;
set F2z = F2 . z;
thus F . z = multcomplex . ((F1 . z),(F2 . z)) by A4, A3, A5, FUNCOP_1:22
.= (F1 . z) * (F2 . z) by BINOP_2:def 5
.= multcomplex . ((F2 . z),(F1 . z)) by BINOP_2:def 5 ; :: thesis:

end;

hence F = multcomplex .: (F2,F1) by A4, A3, FUNCOP_1:21; :: thesis: