Returns the collection element associated with a particular key.
If no element is associated with key, then the behavior of element depends on whether it was called with a default argument: if a default argument was passed, its value is returned; otherwise, an error is signaled.
All collections are required to implement element.
open
| collection | An instance of <collection>. |
| key | An instance of <object>. |
| default: | An instance of <object>. The default is #f. |
| element | An instance of <object>. |