Find an element and return its index.
sealed
| elt | An instance of <object>. Passed as the first argument to test. |
| seq | An instance of <sequence>. |
| test: | An instance of <function>. The test function to find an element. Its signature is (elt, <object>) => (<object>). Defaults to =. |
| failure: | An instance of <object>. The value returned if no matching element is found. Defaults to #f. |
| index | An instance of <integer> or the failure value. The index of the found element. |