input: 
  def construct_mnist_model():
      """Construct a neural network model in Python using Keras to correctly classify the MNIST data set.
      Example:
      for construct_mnist_model() == model
      """
      
      import keras
  
      # Input layer
      input_layer = keras.layers.Input(shape=(784,))
  
      # Hidden layer
      hidden_layer = keras.layers.Dense(128, activation='relu')(input_layer)
  
      # Output layer
      output_layer = keras.layers.Dense(10, activation='softmax')(hidden_layer)
  
      # Model
      model = keras.Model(inputs=input_layer, outputs=output_layer)
  
      # Compile model
      model.compile(optimizer='adam', loss='sparse_categorical_crossentropy', metrics=['accuracy'])
      
      return model
  
output:     return model