reserve S for non empty non void ManySortedSign,
  V for non-empty ManySortedSet of the carrier of S,
  A for non-empty MSAlgebra over S,
  X for non empty Subset of S-Terms V,
  t for Element of X;
reserve S for non empty non void ManySortedSign,
  A for non-empty finite-yielding MSAlgebra over S,
  V for Variables of A,
  X for SetWithCompoundTerm of S,V;

theorem Th26:
  for S1, S2 being non empty ManySortedSign, f,g being Function
  st S1, S2 are_equivalent_wrt f, g holds S2, S1 are_equivalent_wrt f", g"
proof
  let S1, S2 be non empty ManySortedSign, f,g be Function such that
A1: f is one-to-one and
A2: g is one-to-one and
A3: f, g form_morphism_between S1, S2 and
A4: f", g" form_morphism_between S2, S1;
  thus f" is one-to-one & g" is one-to-one by A1,A2;
  f" " = f by A1,FUNCT_1:43;
  hence thesis by A2,A3,A4,FUNCT_1:43;
end;
