consider f being Function such that
A1: ( dom f = F₁() & ( for x being object st x in F₁() holds
f . x = F₂(x) ) ) from FUNCT_1:sch 3();
reconsider p = f as XFinSequence by A1, FINSET_1:10, ORDINAL1:def 7;
take p ; :: thesis: ( len p = F₁() & ( for k being Nat st k in F₁() holds
p . k = F₂(k) ) )
thus ( len p = F₁() & ( for k being Nat st k in F₁() holds
p . k = F₂(k) ) ) by A1; :: thesis: verum