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;
