code: def ylogydu(y, u):
      
      mask = (np.atleast_1d(y)!=0.)
      out = np.zeros_like(u)
      out[mask] = y[mask] * np.log(y[mask] / u[mask])
      return out
docs: tool to give desired output for the limit as y -> 0, which is 0
  
      Parameters
      ----------
      y : array-like of len(n)
      u : array-like of len(n)
  
      Returns
      -------
      np.array len(n)