code: function compile(ast, opts) {
    opts = assign({}, config.defaultOpts, opts)
    debug('compile', opts, ast)
    var worker = makeWorker(opts)
    for (var i = 0, text = ast.value, len = text.length; i < len; i++) {
      compileLine(text[i], worker)
    }
    return worker.buf.join('')
  }
docs: Compile parsed MML-document into HTML.
  @param {object} ast Parsed WPML document
  @param {object} opts Same as opts in mml.doc
  @return{ string} Generated html string.