Here's an implementation of infix notation for Shen; it's effectively Dijkstra's
shunting-yard algorithm. For custom precedence redefine `prec`

.

```
(define prec
** -> 4
* -> 3
/ -> 3
+ -> 2
- -> 2)
\* power is defined in the maths library *\
(define **
X Y -> (power X Y))
(define shunt
[] Output [] -> Output
[] [] [X Op Y | Rest] -> (shunt [Op] [(shunt [] [] Y) (shunt [] [] X)] Rest) where (element? Op [+ - * / **])
[] [] X -> X
[Op | Ops] [Y X] [] -> (shunt Ops [Op X Y] [])
[Op2 | Ops] [Y X | R] [Op1 Z | Rest] ->
(let Nout (if (> (prec Op1) (prec Op2))
[[Op1 Y (shunt [] [] Z)] X | R]
[(shunt [] [] Z) [Op2 X Y] | R])
Nops (if (> (prec Op1) (prec Op2))
[Op2 | Ops]
[Op1 | Ops])
(shunt Nops Nout Rest)) where (element? Op1 [+ - * / **]))
(defmacro infix-macro
[in | X] -> (shunt [] [] X))
```

It can be called as follows:

`(in (2 + 3) * 4)`