input: Write a python function to make predictions based on features using a helper function.
  assert predict([1, 2, 3]) == calculate_feature(1) + calculate_feature(2) + calculate_feature(3)
  assert predict([4, 5]) == calculate_feature(4) + calculate_feature(5)
  assert predict([0, -1, -2]) == calculate_feature(0) + calculate_feature(-1) + calculate_feature(-2)
output: def calculate_feature(x):
      # Dummy implementation for the sake of completeness
      return x * 2
  
  def predict(features):
      return sum(calculate_feature(f) for f in features)