code: def contrast_diff_mean(nm):
      """contrast or restriction matrix for all against mean comparison
  
      Parameters
      ----------
      nm : int
  
      Returns
      -------
      contr : ndarray, 2d, (nm-1, nm)
         contrast matrix for all against mean comparisons
  
      """
      return np.eye(nm) - np.ones((nm, nm)) / nm
  
imports: from numpy.testing import assert_equal
  import numpy
label_desc: Use image node_id 3 for calling a global function with example usage: contrast_diff_mean(nm) and returns: unknown