pub fn left(&self) -> Prec {
        if let Prec::End = self {
            panic!("Can not associate further left")
        }
        return unsafe { mem::transmute(*self as u8 + 1) };
    }
Increments precedence level to cause the
parser to associate infix operators to the left.
For example, addition is left-associated:
```build
Prec::Addition.left()
```
`a + b + c` left-associated becomes `(a + b) + c`.
By default, the parser accociates right.

Panics if you try to associate left on `Prec::End`,
as this is the highest precedence.
passerine/src/compiler/parse.rs