surprising behaviour of map.contains

[divarvel]

The behaviour of map.contains is a bit surprising / inconsistent with .get

• .get() errors when called with something else than a string or int, whereas contains just returns false
• .contains() works with keys, not values (the equivalent behaviour on values can be recovered with .any())

This is part of the spec now, but we might want to revisit that in a future major version with breaking changes