function nestTokens(tokens) {
    var nestedTokens = [];
    var collector = nestedTokens;
    var sections = [];

    var token, section;
    for (var i = 0, numTokens = tokens.length; i < numTokens; ++i) {
      token = tokens[i];

      switch (token[0]) {
      case '#':
      case '^':
        collector.push(token);
        sections.push(token);
        collector = token[4] = [];
        break;
      case '/':
        section = sections.pop();
        section[5] = token[2];
        collector = sections.length > 0 ? sections[sections.length - 1][4] : nestedTokens;
        break;
      default:
        collector.push(token);
      }
    }

    return nestedTokens;
  }
Forms the given array of `tokens` into a nested tree structure where
tokens that represent a section have two additional items: 1) an array of
all tokens that appear in that section and 2) the index in the original
template that represents the end of that section.