Therefore, absolute position (Vaswani et al., 2017) or relative position (Shaw et al., 2018) are generally used to capture the sequential order of words in the sentence.
several researches reveal that the sequential structure may not be sufficient for NLP tasks (Tai et al., 2015;Kim et al., 2017;Shen et al., 2019), since sentences inherently have hierarchical structures (Chomsky, 1965;Bever, 1970).