  Recently, concatenating multiple keyphrases as a target sequence has been
proposed as a new learning paradigm for keyphrase generation. Existing studies
concatenate target keyphrases in different orders but no study has examined the
effects of ordering on models' behavior. In this paper, we propose several
orderings for concatenation and inspect the important factors for training a
successful keyphrase generation model. By running comprehensive comparisons, we
observe one preferable ordering and summarize a number of empirical findings
and challenges, which can shed light on future research on this line of work.
