Currently, the expression hash.contains(5) gets translated to contains(hash).eval(&5). If you understand that context, it makes sense. If you are coming from another language that uses in or contains operators, it gets confusing (the expression 4 < 5 gets translated to: lt(5).eval(&4))

• Some languages have an in operator. So 5 in hash would naturally translate to in(hash).eval(&5).
• Some languages flip that and have a contains operator. So hash contains 5 would naturally translate to contains(5).eval(&hash).