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;
