However, summarization datasets are limited to a single reference summary per document (Sandhaus, 2008; Hermann et al., 2015; Grusky et al., 2018;<cite> Narayan et al., 2018b)</cite> thus evaluations using them is prone to reference bias (Louis and Nenkova, 2013) , also a known issue in machine translation evaluation (Fomicheva and Specia, 2016) .