find the position of an element (float or symbol) in a list, or -1 if it is not found. in case of duplicate elements, it only tells the position of the first occurrence. look for a symbol. look for a float. this has the same gotchas as using [==] and [sel], about the precision of numbers. nonnegative list index of first occurrence, or -1 if not found. don't the list to be searching in