```doc
# Subtraction

Values which wish to implement the `-` infix operator must implement this protocol.
```
protocol Subtraction, as:
  ```doc
  Subtract `rhs` from `lhs`, returning an option.

  It is left up to the implementer to define when it should return `Some` or `None`.
  ```
  def subtract(lhs: Self, rhs: Any): Option
end