Looks up a value given a key. See Associative lists.
| elt | An instance of <object>. The key element to find the value element. |
| seq | An instance of <sequence>. An associative list. |
| key: | An instance of <function>. The function used to obtain the key of an element in the associative list. The default is head. |
| test: | An instance of <function>. The function used to compare keys. The default is =. |
| found | An instance of <object>. |