Duplicate logic in Expression.Evaluator

[gebn]

The -' and +' functions carry out the same functionality with a different operator. Given multiplication and division operations will require similar logic, it makes sense to abstract this out into a single function with a signature along the lines of ∀ {a} {A : Set a} → Maybe A → Maybe A → (A → A → A) → Maybe A, which takes two Maybe types along with an operator, and applies the operator if and only if both parameters are not nothing. It could be called like so:

maybeOp (⟦ E ⟧ σ) (⟦ E' ⟧ σ) _+_ -- for addition
maybeOp (⟦ E ⟧ σ) (⟦ E' ⟧ σ) _∸_ -- for subtraction