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 Th66:
  Free Ex(x,p) = Free p \ {x}
proof
  thus Free Ex(x,p) = Free All(x,'not' p) by Th60
    .= Free 'not' p \ {x} by Th62
    .= Free p \ {x} by Th60;
end;
