nonnegative list index of first occurrence, or -1 if not found. like [tabread], this outputs an element when given an index, but unlike [tabread], this uses a list instead of an array. a [listfind] with the same content cancels the effect of a [listread] when the input of [listread] is nonnegative and is smaller than the list's length. lookup an element. if the index is nonnegative, then it must be less than the list length, else no output will be produced. if the index is negative, then it must be greater than or equal to minus the list length, and this counts from the end of the list: -1 will output the last element, -2 will output the next to last, etc.