`expr  3 \| 1W + 4` fails when it should return 3

With our impl:

```
$ ./target/debug/coreutils expr  3 \| 1W + 4
expr: syntax error (first RPN token does not represent the root of the expression AST)
```
(side note, we could improve our default error message here)

With GNU:

```
/usr/bin/expr  3 \| 1W + 4
3
```