CodeMirror.registerHelper("fold", "markdown", function(cm, start) {
  var maxDepth = 100;

  function isHeader(lineNo) {
    var tokentype = cm.getTokenTypeAt(CodeMirror.Pos(lineNo, 0));
    return tokentype && /\bheader\b/.test(tokentype);
  }

  function headerLevel(lineNo, line, nextLine) {
    var match = line && line.match(/^#+/);
    if (match && isHeader(lineNo)) return match[0].length;
    match = nextLine && nextLine.match(/^[=\-]+\s*$/);
    if (match && isHeader(lineNo + 1)) return nextLine[0] == "=" ? 1 : 2;
    return maxDepth;
  }

  var firstLine = cm.getLine(start.line), nextLine = cm.getLine(start.line + 1);
  var level = headerLevel(start.line, firstLine, nextLine);
  if (level === maxDepth) return undefined;

  var lastLineNo = cm.lastLine();
  var end = start.line, nextNextLine = cm.getLine(end + 2);
  while (end < lastLineNo) {
    if (headerLevel(end + 1, nextLine, nextNextLine) <= level) break;
    ++end;
    nextLine = nextNextLine;
    nextNextLine = cm.getLine(end + 2);
  }

  return {
    from: CodeMirror.Pos(start.line, firstLine.length),
    to: CodeMirror.Pos(end, cm.getLine(end).length)
  };
});
