'A function that groups the given values.
Override this function to introduce custom grouping logic. The default
implementation assumes each value belongs in its own group.
Args:
prop: The name of the property who\'s values are being grouped.
values: A list of opaque values.
Returns:
A list of lists of grouped values.'

def _group_values(self, prop, values):

    return [[value] for value in values]
