code: function format (form) {
          return formToMarkup(form)
          // Split lines.
          .split('\n')
          .map(function (line, index) {
            return EMPTY_LINE.test(line)
            // If the line is empty, leave it empty.
            ? line
            : index === 0
                ? line
                // On subsequent lines with indentation, add spaces
                // to existing indentation.
                : ' '.repeat(token.position.column - 1) + line
          })
          .join('\n')
        }
docs: Format markup for insertion at the current position.