reserve a,b,c,d,x,y,z for object, X,Y,Z for set;
reserve R,S,T for Relation;
reserve F,G for Function;

theorem Th42:
  R,S are_isomorphic & S,T are_isomorphic implies R,T are_isomorphic
proof
  given F such that
A1: F is_isomorphism_of R,S;
  given G such that
A2: G is_isomorphism_of S,T;
  take G*F;
  thus thesis by A1,A2,Th41;
end;
