reserve S for non void non empty ManySortedSign,
  U1,U2 for MSAlgebra over S,
  o for OperSymbol of S,
  n for Nat;

theorem Th15:
  for U1,U2,U3 being non-empty MSAlgebra over S for H be
  ManySortedFunction of U1,U2, H1 be ManySortedFunction of U2,U3 st H
is_isomorphism U1,U2 & H1 is_isomorphism U2,U3 holds H1 ** H is_isomorphism U1,
  U3
by Th11,Th12;
