code: def _validate_mathtext_fallback(s):
      _fallback_fonts = ["cm", "stix", "stixsans"]
      if isinstance(s, str):
          s = s.lower()
      if s is None or s == "none":
          return None
      elif s.lower() in _fallback_fonts:
          return s
      else:
          raise ValueError(
              f"{s} is not a valid fallback font name. Valid fallback font "
              f"names are {','.join(_fallback_fonts)}. Passing 'None' will turn "
              "fallback off."
          )
  
imports: import ast
  from functools import lru_cache, reduce
  from numbers import Real
  import operator
  import os
  import re
  import numpy
  from matplotlib import _api, cbook
  from matplotlib.cbook import ls_mapper
  from matplotlib.colors import Colormap, is_color_like
  from matplotlib._fontconfig_pattern import parse_fontconfig_pattern
  from matplotlib._enums import JoinStyle, CapStyle
  from cycler import Cycler, cycler
label_desc: Use image node_id 24 for calling a global function with example usage: _validate_mathtext_fallback(s) and returns: None, s