大方廣圓覺修多羅了義經略疏 2.1383

2.1383

凡全偈者，所謂四句。句有二種，謂文句、義句。若約文句，即兩句為半偈；若約義句，即說一切法本來空寂是半偈也，顯空體不空方為全偈。或無常、真常為半及全。

In general, a complete gāthā consists of four lines. Lines are of two kinds: textual lines and doctrinal lines. If viewed in terms of textual lines, then two lines make half a gāthā. If viewed in terms of doctrinal lines, then stating that all phenomena are originally empty and quiescent is half a gāthā; revealing that the substance of emptiness is not empty only then constitutes a complete gāthā. Or impermanence and true permanence may constitute the half and the complete respectively.