['But', 'they', 'usually', 'made', 'the', 'assumption', 'that', 'it', 'is', 'known', 'a', 'priori', 'that', 'the', 'system', 'under', 'study', 'can', 'be', 'represented', 'by', 'just', 'two', 'or', 'three', 'terms', '.']