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 Th1:
  Var1 (x '=' y) = x & Var2 (x '=' y) = y
proof
  x '=' y is being_equality;
  then x '=' y = (Var1 (x '=' y)) '=' (Var2 (x '=' y)) by ZF_LANG:36;
  hence thesis by ZF_LANG:1;
end;
