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 Th2:
  Var1 (x 'in' y) = x & Var2 (x 'in' y) = y
proof
  x 'in' y is being_membership;
  then x 'in' y = (Var1 (x 'in' y)) 'in' (Var2 (x 'in' y)) by ZF_LANG:37;
  hence thesis by ZF_LANG:2;
end;
