reserve X for set, R,R1,R2 for Relation;
reserve x,y,z for set;
reserve n,m,k for Nat;

theorem
  for R being Relation of X holds field R c= X
  proof
    let R be Relation of X;
    dom R c= X & rng R c= X;
    hence field R c= X by XBOOLE_1:8;
  end;
