let S be non empty non void ManySortedSign ; :: thesis: for U1, U2 being MSAlgebra over S
for F being ManySortedFunction of U1,U2 holds
( F is_isomorphism U1,U2 iff ( F is_homomorphism U1,U2 & F is "onto" & F is "1-1" ) )

let U1, U2 be MSAlgebra over S; :: thesis: for F being ManySortedFunction of U1,U2 holds
( F is_isomorphism U1,U2 iff ( F is_homomorphism U1,U2 & F is "onto" & F is "1-1" ) )

let F be ManySortedFunction of U1,U2; :: thesis: ( F is_isomorphism U1,U2 iff ( F is_homomorphism U1,U2 & F is "onto" & F is "1-1" ) )
thus ( F is_isomorphism U1,U2 implies ( F is_homomorphism U1,U2 & F is "onto" & F is "1-1" ) ) :: thesis: ( F is_homomorphism U1,U2 & F is "onto" & F is "1-1" implies F is_isomorphism U1,U2 )
proof
assume F is_isomorphism U1,U2 ; :: thesis: ( F is_homomorphism U1,U2 & F is "onto" & F is "1-1" )
then ( F is_epimorphism U1,U2 & F is_monomorphism U1,U2 ) ;
hence ( F is_homomorphism U1,U2 & F is "onto" & F is "1-1" ) ; :: thesis: verum
end;
assume ( F is_homomorphism U1,U2 & F is "onto" & F is "1-1" ) ; :: thesis: F is_isomorphism U1,U2
then ( F is_epimorphism U1,U2 & F is_monomorphism U1,U2 ) ;
hence F is_isomorphism U1,U2 ; :: thesis: verum