Furthermore, as the newly proposed architecture called the Transformer network<cite> (Vaswani et al., 2017)</cite> has shown to be effective in various sequence-to-sequence problems, various multi-source adaptations of the Transformer (Junczys-Dowmunt and Grundkiewicz, 2018; Shin and Lee, 2018; Tebbifakhr et al., 2018) have been applied to APE.