Returns #t if a key is in a collection. If the key is in the collection, then the second value is the element associated with key; otherwise, the second return value is an undefined value.
These two implementations are distinct, but have the same effect.
sealed
| collection | An instance of <collection>. |
| key | An instance of <object>. |
| key-exists? | An instance of <boolean>. |
| value | An instance of <object>. The associated datum for the key. |