def get_generated_cols(X_original, X_transformed, to_transform):
    """
    Returns a list of the generated/transformed columns.

    Arguments:
        X_original: df
            the original (input) DataFrame.
        X_transformed: df
            the transformed (current) DataFrame.
        to_transform: [str]
            a list of columns that were transformed (as in the original DataFrame), commonly self.cols.

    Output:
        a list of columns that were transformed (as in the current DataFrame).
    """
    original_cols = set(X_original.columns)
    current_cols = set(X_transformed.columns)
    generated_cols = list(current_cols - (original_cols - set(to_transform)))

    return generated_cols