reserve p,p1,p2,q,r,F,G,G1,G2,H,H1,H2 for ZF-formula,
  x,x1,x2,y,y1,y2,z,z1,z2,s,t for Variable,
  a,X for set;

theorem
  Free (p 'or' q) = Free p \/ Free q
proof
  thus Free (p 'or' q) = Free ('not' p '&' 'not' q) by Th60
    .= Free 'not' p \/ Free 'not' q by Th61
    .= Free p \/ Free 'not' q by Th60
    .= Free p \/ Free q by Th60;
end;
