vendors/tempy.js/expand
    function expand(str, isTop) {
      var expansions = [];
      var m = balanced("{", "}", str);
      if (!m || /\$$/.test(m.pre))
        return [str];
      var isNumericSequence = /^-?\d+\.\.-?\d+(?:\.\.-?\d+)?$/.test(m.body);
      var isAlphaSequence = /^[a-zA-Z]\.\.[a-zA-Z](?:\.\.-?\d+)?$/.test(m.body);
      var isSequence = isNumericSequence || isAlphaSequence;
      var isOptions = m.body.indexOf(",") >= 0;
      if (!isSequence && !isOptions) {
        if (m.post.match(/,.*\}/)) {
          str = m.pre + "{" + m.body + escClose + m.post;
          return expand(str);
        }
        return [str];
      }
      var n;
      if (isSequence) {
        n = m.body.split(/\.\./);
      } else {
        n = parseCommaParts(m.body);
        if (n.length === 1) {
          n = expand(n[0], false).map(embrace);
          if (n.length === 1) {
            var post = m.post.length ? expand(m.post, false) : [""];
            return post.map(function(p) {
              return m.pre + n[0] + p;
            });
          }
        }
      }
      var pre = m.pre;
      var post = m.post.length ? expand(m.post, false) : [""];
      var N;
      if (isSequence) {
        var x = numeric(n[0]);
        var y = numeric(n[1]);
        var width = Math.max(n[0].length, n[1].length);
        var incr = n.length == 3 ? Math.abs(numeric(n[2])) : 1;
        var test = lte;
        var reverse = y < x;
        if (reverse) {
          incr *= -1;
          test = gte;
        }
        var pad = n.some(isPadded);
        N = [];
        for (var i = x; test(i, y); i += incr) {
          var c;
          if (isAlphaSequence) {
            c = String.fromCharCode(i);
            if (c === "\\")
              c = "";
          } else {
            c = String(i);
            if (pad) {
              var need = width - c.length;
              if (need > 0) {
                var z = new Array(need + 1).join("0");
                if (i < 0)
                  c = "-" + z + c.slice(1);
                else
                  c = z + c;
              }
            }
          }
          N.push(c);
        }
      } else {
        N = concatMap(n, function(el) {
          return expand(el, false);
        });
      }
      for (var j = 0; j < N.length; j++) {
        for (var k = 0; k < post.length; k++) {
          var expansion = pre + N[j] + post[k];
          if (!isTop || isSequence || expansion)
            expansions.push(expansion);
        }
      }
      return expansions;
    }