theorem EquivTransitive:
  a,b equiv p,q & p,q equiv r,s implies a,b equiv r,s
  proof
    assume that
X1: a,b equiv p,q and
X2: p,q equiv r,s;
    p,q equiv a,b by X1, EquivSymmetric;
    hence thesis by X2, A2;
  end;
