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

theorem Th11:
  R1\,R2 c= R1 \/ R2
  proof
    let x,y be object;
    reconsider xx = x, yy = y as set by TARSKI:1;
    assume [x,y] in R1\,R2; then
    xx,yy in R1\,R2; then
    xx,yy in R1 or xx,yy in R2 by Th9; then
    [x,y] in R1 or [x,y] in R2;
    hence thesis by XBOOLE_0:def 3;
  end;
