code: def child(idx, dim, axis):
      
      idxm = multi_index(idx, dim)
      out = numpy.array(idxm) + 1*(numpy.eye(len(idxm))[axis])
      return single_index(out)
docs: Child node according to Bertran's notation.
  
      Parameters
      ----------
      idx : int
          Index of the parent node.
      dim : int
          Dimensionality of the problem.
      axis : int
          Dimension direction to define a child.
          Must have `0<=axis<dim`
  
      Returns
      -------
      out : int
          Index of child node with `out > idx`.
  
      Examples
      --------
      >>> print(chaospy.bertran.child(4, 1, 0))
      5
      >>> print(chaospy.bertran.child(4, 2, 1))
      8