Add an attribute value to the given vertices.

        :param int value: Attribute value.
        :param dict att_mappings: Dictionary of mappings between vertices and enumerated attributes.
        :param list indices: Indices of the vertices.

def _add_attribute_values(self, value, att_mappings, indices):
        """Add an attribute value to the given vertices.

        :param int value: Attribute value.
        :param dict att_mappings: Dictionary of mappings between vertices and enumerated attributes.
        :param list indices: Indices of the vertices.
        """
        for i in indices:
            att_mappings[i].append(value)