Natyashastra
यस्माच्च लोकपालानां परिवृत्य चतुर्दिशम् । वन्दनानि प्रकुर्वन्ति तस्माच्च परिवर्तनम् ॥ २३॥
Since the guardians of the world (Lokapalas) are surrounded on all four directions and perform salutations, therefore, the change (or transformation) is performed accordingly.
english translation
yasmAcca lokapAlAnAM parivRtya caturdizam । vandanAni prakurvanti tasmAcca parivartanam ॥ 23॥
hk transliteration by Sanscript