theorem Th51:
 canonical_homomorphism T is x-constant
  proof set H = canonical_homomorphism T;
    thus H.(x-term) = x-term;
    let s1; let x1 be Element of X.s1;
    assume x1 <> x or s <> s1;
    then x1-term is x-omitting by ThC1;
    hence thesis;
  end;
