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;
