code: def gradient_shell(x):
      r = ((x - 0.5) ** 2).sum()
      # second term gives the vector pointing to the center
      # third term is positive if r > 0.4, negative otherwise
      # v = -4 * (x - 0.5) * ((r - 0.4))**3
      # v /= (v**2).sum()**0.5
  
      # simplified:
      v = gradient_to_center(x)
      if r < 0.4:
          # point outwards if inside
          v *= -1
  
      return v
  
imports: import numpy
  from math import gamma, pi, exp
label_desc: Use image node_id 20 for calling a global function with example usage: gradient_shell(x) and returns: v