 With the introduction of Tang poetry to Korea in the mid-Joseon Period, "hanmun" poetry began making significant strides as an art form.Traditional Tang poetry ("koshi") was more formulaic and imposed prescriptive tonal guidelines.During the lifetime of Nanseolheon, new forms of poetry that incorporated tonal irregularities, lines with non-standard syllable counts, and length (broadly referred to as "kunch'e shi", of which "yulshi" and "cheolgu" are subsets) began to come into favor.