eval x=1; y=2; y+x;

eval x=1; y=2; y+x;

eval x=1; y=2; y+x;

      root
       |
      prog
 ______|______
 |     |     |
 =     =     +
_|__  _|__  _|__
|  |  |  |  |  |
x  1  y  2  y  x
op is =
===========================
      root
       |
      prog
 ______|______
 |     |     |
 =     =     +
_|__  _|__  _|__
|  |  |  |  |  |
x  1  y  2  y  x
---------------------------
op is =
===========================
      root
       |
      prog
 ______|______
 |     |     |
 =     =     +
_|__  _|__  _|__
|  |  |  |  |  |
x  1  y  2  y  x
---------------------------
eval name n: 2
eval name n: 1
eval name n: 2
eval name n: 1
ret: 3, n1: 2, n2: 1
===========================
      root
       |
      prog
 ______|______
 |     |     |
 =     =     3
_|__  _|__
|  |  |  |
x  1  y  2
---------------------------
===========================
      root
       |
      prog
 ______|______
 |     |     |
 =     =     3
_|__  _|__
|  |  |  |
x  1  y  2
---------------------------