釋鑑稽古略續集 3.105

3.105

辛未二年　壬申三年。李賢進戰車之議。欲行則行。欲止則止。謂之有脚之城。備邊長策有所取焉。

The xinwei second year. The renshen third year: Li Xian presented a proposal for war chariots, which could move when desired and halt when desired, calling them "castles with feet." This was adopted as a long-term strategy for border defense.