code: def nmb_weights_hidden(self) -> int:
          
          nmb = 0
          for idx_layer in range(self.nmb_layers-1):
              nmb += self.nmb_neurons[idx_layer] * self.nmb_neurons[idx_layer+1]
          return nmb
docs: Number of hidden weights.
  
          >>> from hydpy import ANN
          >>> ann = ANN(None)
          >>> ann(nmb_inputs=2, nmb_neurons=(4, 3, 2), nmb_outputs=3)
          >>> ann.nmb_weights_hidden
          18