<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def where(self, field_path, op_string, value):
 """Create a "where" query with this collection as parent. See :meth:`~.firestore_v1beta1.query.Query.where` for more information on this method. Args: field_path (str):
 A field path (``.``-delimited list of field names) for the field to filter on. op_string (str):
 A comparison operation in the form of a string. Acceptable values are ``<``, ``<=``, ``==``, ``>=`` and ``>``. value (Any):
 The value to compare the field against in the filter. If ``value`` is :data:`None` or a NaN, then ``==`` is the only allowed operation. Returns: ~.firestore_v1beta1.query.Query: A filtered query. """

        query = query_mod.Query(self)
        return query.where(field_path, op_string, value)