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 Th40:
  R,S are_isomorphic implies S,R are_isomorphic
proof
  given F such that
A1: F is_isomorphism_of R,S;
  take F";
  thus thesis by A1,Th39;
end;
