def prepareDataForModelSelection(X_train, y_train, start_validation):
    """
    gets train set and generates a validation set splitting the train.
    The validation set is mandatory for feature and model selection.
    """
    X = X_train[X_train.index < start_validation]
    y = y_train[y_train.index < start_validation]    
    
    X_val = X_train[X_train.index >= start_validation]    
    y_val = y_train[y_train.index >= start_validation]   
    
    return X, y, X_val, y_val