let F, f be Function; :: thesis: for i, xi being set st xi in F . i & f in product F holds
f +* (i,xi) in product F

let i, xi be set ; :: thesis: ( xi in F . i & f in product F implies f +* (i,xi) in product F )
assume A1: xi in F . i ; :: thesis: ( not f in product F or f +* (i,xi) in product F )
assume A2: f in product F ; :: thesis: f +* (i,xi) in product F
A3: for x being object st x in dom F holds
(f +* (i,xi)) . x in F . x
proof
let x be object ; :: thesis: ( x in dom F implies (f +* (i,xi)) . x in F . x )
assume A4: x in dom F ; :: thesis: (f +* (i,xi)) . x in F . x
per cases ( i = x or i <> x ) ;
suppose A5: i = x ; :: thesis: (f +* (i,xi)) . x in F . x
thus (f +* (i,xi)) . x in F . x :: thesis: verum
proof
per cases ( i in dom f or not i in dom f ) ;
suppose i in dom f ; :: thesis: (f +* (i,xi)) . x in F . x
hence (f +* (i,xi)) . x in F . x by ; :: thesis: verum
end;
suppose not i in dom f ; :: thesis: (f +* (i,xi)) . x in F . x
then f +* (i,xi) = f by FUNCT_7:def 3;
hence (f +* (i,xi)) . x in F . x by ; :: thesis: verum
end;
end;
end;
end;
suppose i <> x ; :: thesis: (f +* (i,xi)) . x in F . x
then (f +* (i,xi)) . x = f . x by FUNCT_7:32;
hence (f +* (i,xi)) . x in F . x by ; :: thesis: verum
end;
end;
end;
dom f = dom F by ;
then dom (f +* (i,xi)) = dom F by FUNCT_7:30;
hence f +* (i,xi) in product F by ; :: thesis: verum