code: def _msk_from_awf_error(f, msk):
      """Helper function that calculates the error of a predicted mean signal
      kurtosis from the axonal water fraction of SMT2 model and a measured
      mean signal kurtosis
  
      Parameters
      ----------
      f : float
          Axonal volume fraction estimate.
      msk : float
          Measured mean signal kurtosis.
  
      Returns
      -------
      error : float
         Error computed by subtracting msk with fun(f), where fun is the function
         described in equation 17 of [1]_
  
      Notes
      -----
      This function corresponds to the differential of equations 17 of [1]_
      """
      return msk_from_awf(f) - msk
  
imports: import numpy
  import scipy.optimize
  from dipy.core.gradients import check_multi_b, unique_bvals_magnitude, round_bvals
  from dipy.reconst.base import ReconstModel
  from dipy.reconst.dti import MIN_POSITIVE_SIGNAL
  from dipy.core.ndindex import ndindex
  from dipy.core.onetime import auto_attr
label_desc: Use image node_id 3 for calling a global function with example usage: _msk_from_awf_error(f, msk) and returns: unknown