The headword verb is second person singular imperative of proi+/sthmi; see pi 2816 and pi 2817 for forms of the same verb, pi 2805 and pi 2806 for related verbs, and pi 2802, pi 2803, pi 2807 and pi 2808 for related nouns. Adler compares this entry (lacking, she reports, in mss AFV) to entries in the Lexicon Syntacticum Vaticanum 93 and the Lexicon Syntacticum of codex Laurentianus 59.16. The headword phrase as such, which might simply be paradigmatic, is attested elsewhere only in Ephraem the Syrian, Prayer to the Mother of God 6.
[1] For an instance, see pi 2811.