code: function findDoubleVacancies(str, size) {
      var vacs = [];
      for (var i = 0; i < CANDIDATE_CHARS.length; i++) {
          for (var j = 0; j < CANDIDATE_CHARS.length; j++) {
              if (i != j) {
                  var pair = CANDIDATE_CHARS[i] + CANDIDATE_CHARS[j];
                  if (str.indexOf(pair) < 0) {
                      vacs.push(pair);
                      if (size && vacs.length >= size) break;
                  }
              }
          }
      }
      return vacs;
  }
docs: Find two-character vacancies of a string; currently unused
  @param {string} str
  @param {number} [size]
  @returns {string[]}