reserve S,S9 for non void Signature,
  f,g for Function;

theorem
  for S being non void Signature, A being feasible MSAlgebra over S
  holds A is Algebra of S
proof
  let S be non void Signature, A be feasible MSAlgebra over S;
A1: S is Extension of S by Th45;
  A is Algebra by Def6;
  hence thesis by A1,Def7;
end;
