Return a JSON representation of the sync map.

        :rtype: string

        .. versionadded:: 1.3.1

def json_string(self):
        """
        Return a JSON representation of the sync map.

        :rtype: string

        .. versionadded:: 1.3.1
        """
        def visit_children(node):
            """ Recursively visit the fragments_tree """
            output_fragments = []
            for child in node.children_not_empty:
                fragment = child.value
                text = fragment.text_fragment
                output_fragments.append({
                    "id": text.identifier,
                    "language": text.language,
                    "lines": text.lines,
                    "begin": gf.time_to_ssmmm(fragment.begin),
                    "end": gf.time_to_ssmmm(fragment.end),
                    "children": visit_children(child)
                })
            return output_fragments
        output_fragments = visit_children(self.fragments_tree)
        return gf.safe_unicode(
            json.dumps({"fragments": output_fragments}, indent=1, sort_keys=True)
        )