Implement identity, binary operator, and skipping operator for associative commutative operators

[niklasdewally]

Description

For every AC operator we could have a triplet defined: identity, binary operator, skipping operator. The first two are hopefully clear, the third is a bit weird but useful.

Let's take sum as an example. 0 is the identity, + is the binop, \ (b, x) -> toInt(b) * x is the skipping operator.

Skipping op evaluates to identity if b is false, to the second argument if b is true.

For logical and it is \ (b, x) -> b => x

For logical or it is \ (b, x) -> b /\ x

Originally posted by @ozgurakgun in #491 (comment)

Tasks

• Implement is_ac_operator method (or another way to tell if an operator is AC) #494
• Implement identity function for AC operators
• Implement binop function for AC operators
• Implement skipping op function for AC operators

[niklasdewally]

This will become useful later when I get to quantifiers and comprehensions.