theorem Th56:
  Hom(T,x1,x2) = Hom(T,x2,x1)
  proof
    Hom(T,x2,x1).s.(x1-term) = x2-term & Hom(T,x2,x1).s.(x2-term) = x1-term &
    for s1 for y being Element of X.s1 st s1 <> s or y <> x1 & y <> x2
    holds Hom(T,x2,x1).s1.(y-term) = y-term by HOM;
    hence thesis by HOM;
  end;
