The number of parameters a function takes.

```
// each of these is a number in ordinal form
const nullary = () => {}
const unary = x => x
const binary = (a, b) => [a, b]
const ternary = (a, b, c) => ({a, b, c})
const quaternary = (a, b, c, d) => "the name of 5 parameters is toomanyary"
```

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