Setsuwa (anecdotal) literature:Nihon ryōikitoKokon chomonjū 論文集所収 Haruo Shirane ·2015-07-31 ·『Cambridge University Press eBooks』 pp. 280-286 doi:10.1017/cho9781139245869.030
The Acquisition of Japanese Modality during Study Abroad 論文集所収 Suwako Watanabe・Noriko Iwasaki ·2009-01-01 ·『Palgrave Macmillan UK eBooks』 pp. 231-258 doi:10.1057/9780230245754_10