Serves the same purpose as find-key. Locates an element in seq that returns a true value from pred, and returns its key.
sealed
| pred | An instance of <function>. The test function to find an element. |
| seq | An instance of <sequence>. |
| 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. |