class_docstr: Abstract base class for classes used to estimate a function's first
  derivatives, and deduce the dofs for a CubicTriInterpolator using a
  reduced HCT element formulation.
  
  Derived classes implement ``compute_df(self, **kwargs)``, returning
  ``np.vstack([dfx, dfy]).T`` where ``dfx, dfy`` are the estimation of the 2
  gradient coordinates.
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code: def compute_dz(self, **kwargs):
      raise NotImplementedError
  
imports: import numpy
  from matplotlib import _api
  from matplotlib.tri import Triangulation
  from matplotlib.tri._trifinder import TriFinder
  from matplotlib.tri._tritools import TriAnalyzer
label_desc: Use image node_id 2 for calling the _DOF_estimator obj's underlying member method code with example usage: obj.compute_dz() without return types