docstr: Test that we don't blow up when trying to call week_start's
  should_trigger on the first day of a trading environment.
code: def test_NthTradingDayOfWeek_day_zero(self):
      """
      Test that we don't blow up when trying to call week_start's
      should_trigger on the first day of a trading environment.
      """
      rule = NthTradingDayOfWeek(0)
      rule.cal = self.cal
      first_open = self.cal.open_and_close_for_session(
          self.cal.all_sessions[0]
      )
      self.assertTrue(first_open)
  
imports: import datetime
  from inspect import isabstract
  import random
  from unittest import TestCase
  import warnings
  from nose_parameterized import parameterized
  import pandas
  from six import iteritems
  from six.moves import range, map
  import catalyst.utils.events
  from catalyst.utils.calendars import get_calendar
  from catalyst.utils.events import EventRule, StatelessRule, Always, Never, AfterOpen, ComposedRule, BeforeClose, NotHalfDay, NthTradingDayOfWeek, NDaysBeforeLastTradingDayOfWeek, NthTradingDayOfMonth, NDaysBeforeLastTradingDayOfMonth, StatefulRule, OncePerDay, _build_offset, _build_date, _build_time, EventManager, Event, MAX_MONTH_RANGE, MAX_WEEK_RANGE, TradingDayOfMonthRule, TradingDayOfWeekRule
label_desc: Use image node_id 8 for calling the StatelessRulesTests obj's underlying member method code with example usage: obj.test_NthTradingDayOfWeek_day_zero() without return types