function countSubstringsL(k, s, l) {
  return (((Math.pow(k, 2) - Math.pow(Math.abs(s - l), 2)) / 2) | 0) + k + 1;
}
Function returning the number of substrings that will be selected by the
multi-match-aware selection scheme for theshold `k`, for a string of length
`s` to match strings of length `l`.

@param   {number} k - Levenshtein distance threshold.
@param   {number} s - Length of target strings.
@param   {number} l - Length of strings to match.
@returns {number}   - The number of selected substrings.