docs: function checkPositionsParameter(positions, method) {
          var err;
          if ('undefined' === typeof positions) return;
          if ('number' === typeof positions) {
              if (isNaN(positions) ||
                  positions < 0 ||
                  !isFinite(positions)) {
                  err = true;
              }
              else {
                  positions += '';
              }
          }
  
          if (err || 'string' !== typeof positions || positions.trim() === '') {
              throw new TypeError('Stager.' + method + ': positions must ' +
                                  'be a non-empty string, a positive finite ' +
                                  'number, or undefined. Found: ' + positions);
          }
          return positions;
      }