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```
(x (a b) (c d))
```
```
(x (a b) (c d))
```
```
_*_ 4
_+_ 3
x * y + 2
```
```
(+ (* x y) 2)
```
```
_*_ 4 (adjacent)
3xy
```
```
(* (* 3 x) y)
```
```
_*_ 4 (adjacent)
_+_ 3
x(y + 2)
```
```
(* x (+ y 2))
```