let t1 = n(NT_LET, id("x"), id("y"));
  let t2 = n(NT_ASSIGN, id("x"), n(NT_NUMBER, 111));
  let t3 = n(NT_LET, id("x"), id("y"));
  let t4 = n(NT_ASSIGN, id("x"), n(NT_NUMBER, 222));
  let t5 = n(NT_SEQ, t3, t4);
  let t4 = n(NT_ASSIGN, id("x"), n(NT_NUMBER, 333));
  let t6 = n(NT_SEQ, t1, t2, t5, t4);

  /* 
    equivalent to
      { let x, y;
        x := 111;
        { let x, y;
          x := 222 }
        x := 333 } 
  */