code: def is_subperiod(source, target) -> bool:
      """
      Returns True if downsampling is possible between source and target
      frequencies
  
      Parameters
      ----------
      source : str or DateOffset
          Frequency converting from
      target : str or DateOffset
          Frequency converting to
  
      Returns
      -------
      bool
      """
  
      if target is None or source is None:
          return False
      source = _maybe_coerce_freq(source)
      target = _maybe_coerce_freq(target)
  
      if _is_annual(target):
          if _is_quarterly(source):
              return _quarter_months_conform(
                  get_rule_month(source), get_rule_month(target)
              )
          return source in {
              "D",
              "C",
              "B",
              "M",
              "H",
              "T",
              "S",
              "L",
              "U",
              "N",
          }
      elif _is_quarterly(target):
          return source in {
              "D",
              "C",
              "B",
              "M",
              "H",
              "T",
              "S",
              "L",
              "U",
              "N",
          }
      elif _is_monthly(target):
          return source in {"D", "C", "B", "H", "T", "S", "L", "U", "N"}
      elif _is_weekly(target):
          return source in {
              target,
              "D",
              "C",
              "B",
              "H",
              "T",
              "S",
              "L",
              "U",
              "N",
          }
      elif target == "B":
          return source in {"B", "H", "T", "S", "L", "U", "N"}
      elif target == "C":
          return source in {"C", "H", "T", "S", "L", "U", "N"}
      elif target == "D":
          return source in {"D", "H", "T", "S", "L", "U", "N"}
      elif target == "H":
          return source in {"H", "T", "S", "L", "U", "N"}
      elif target == "T":
          return source in {"T", "S", "L", "U", "N"}
      elif target == "S":
          return source in {"S", "L", "U", "N"}
      elif target == "L":
          return source in {"L", "U", "N"}
      elif target == "U":
          return source in {"U", "N"}
      elif target == "N":
          return source in {"N"}
      else:
          return False
  
imports: from __future__ import annotations
  import numpy
  from pandas._libs.algos import unique_deltas
  from pandas._libs.tslibs import Timestamp, get_unit_from_dtype, periods_per_day, tz_convert_from_utc
  from pandas._libs.tslibs.ccalendar import DAYS, MONTH_ALIASES, MONTH_NUMBERS, MONTHS, int_to_weekday
  from pandas._libs.tslibs.fields import build_field_sarray, month_position_check
  from pandas._libs.tslibs.offsets import DateOffset, Day, to_offset
  from pandas._libs.tslibs.parsing import get_rule_month
  from pandas._typing import npt
  from pandas.util._decorators import cache_readonly
  from pandas.core.dtypes.common import is_datetime64_dtype, is_numeric_dtype, is_period_dtype, is_timedelta64_dtype
  from pandas.core.dtypes.generic import ABCIndex, ABCSeries
  from pandas.core.algorithms import unique
label_desc: Use image node_id 5 for calling a global function with example usage: is_subperiod(source, target) and returns: False, unknown, _quarter_months_conform, unknown, unknown, unknown, unknown, unknown, unknown, unknown, unknown, unknown, unknown, unknown, unknown, False