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
  Ex(x,y,p) is existential & bound_in Ex(x,y,p) = x & the_scope_of Ex(x,
  y,p) = Ex(y,p) by Th9;
