code: def _inertia(i, j, Features):
      """
      Compute the variance of the set which is
      the concatenation of Feature[i] and Features[j]
      """
      n = Features[0][i] + Features[0][j]
      s = Features[1][i] + Features[1][j]
      q = Features[2][i] + Features[2][j]
      return np.sum(q - (s**2 / n))
  
imports: from warnings import warn
  import numpy
  from ..graph.forest import Forest
  from ..graph.graph import WeightedGraph
label_desc: Use image node_id 5 for calling a global function with example usage: _inertia(i, j, Features) and returns: np