docs: def get_edge_by_two_vertices(self, vertex1, vertex2, key=None):
          """ Returns an instance of :class:`bg.edge.BBGEdge` edge between to supplied vertices (if ``key`` is supplied, returns a :class:`bg.edge.BBGEdge` instance about specified edge).
  
          Proxies a call to :meth:`BreakpointGraph._BreakpointGraph__get_edge_by_two_vertices`.
  
          :param vertex1: first vertex instance out of two in current :class:`BreakpointGraph`
          :type vertex1: any hashable object
          :param vertex2: second vertex instance out of two in current :class:`BreakpointGraph`
          :type vertex2: any hashable object
          :param key: unique identifier of edge of interested to be retrieved from current :class:`BreakpointGraph`
          :type key: any python object. ``None`` or ``int`` is expected
          :return: edge between two specified edges respecting a ``key`` argument.
          :rtype: :class:`bg.edge.BGEdge`
          """
          return self.__get_edge_by_two_vertices(vertex1=vertex1, vertex2=vertex2, key=key)