code: def _check_reduction_strategy(strategy: str):
      """Check that the provided strategy is supported.
  
      Parameters
      ----------
      strategy : :obj:`str`
          The name of a valid function to reduce the region with.
          Must be one of: sum, mean, median, minimum, maximum, variance,
          standard_deviation.
  
      """
      available_reduction_strategies = {
          "mean",
          "median",
          "sum",
          "minimum",
          "maximum",
          "standard_deviation",
          "variance",
      }
  
      if strategy not in available_reduction_strategies:
          raise ValueError(
              f"Invalid strategy '{strategy}'. "
              f"Valid strategies are {available_reduction_strategies}."
          )
  
file_path: nilearn/nilearn/regions/signal_extraction.py
imports: import warnings
  import numpy
  from nibabel import Nifti1Image
  from scipy import linalg, ndimage
  from ..None import _utils, masking
  from .._utils.niimg import safe_get_data
  from ..image import new_img_like
label_desc: Use image node_id 5 for calling a global function with example usage: _check_reduction_strategy(strategy) without return types