स्त्यः प्रपूर्वस्य

The verb स्त्या (स्त्यै and ष्ठ्यै) when preceded by प्र changes its semi-vowel to a vowel, when a निष्ठ affix follows.
The phrases "when the nishtha follows", and "there is vocalisation" are understood here. The anuvritti of स्फी does not run here. The roots स्त्यै and ष्ठ्यै both assume the form स्त्या and are included here. Thus प्र + स्त्या + क्त = प्र + स्ति + त 6.1.108 = प्रस्तीतः 6.4.2, and प्रस्तीतवान् ॥ The त of nishtha affix would have been changed into न् by 8.2.43 in as much as the root स्त्या has a semivowel and ends in long आ ॥ But by the vocalisation of य, the condition of यण्वत् for the application of 8.2.43, no longer existing, the affix त is not changed to न ॥ But त is optionally changed to म by 8.2.54 as प्रस्तीमः and प्रस्तीमवान् ॥

Why do we say "when preceded by प्र?" Observe संस्त्यानः 8.2.43, संस्त्यानवान् ॥ If it was intended that vocalisation should take place when प्र singly stood before, then the sutra could well have run thus प्रस्त्यः ॥ The use of the word पुर्वस्य implies that the rule applies when प्र stands first, though other upasargas may intervene between it and the root. Thus प्रसंस्तीतः, प्रसंस्तीतवान् ॥ The compound प्रपूर्वस्य should be explained as a Bahuvrihi= प्रः पूर्वो यस्य धातूपसर्गसमुदायस्य स प्रपूर्व (that which consisting of root and upasarga is preceded by प्र is called प्रपूर्व). Therefore the rule is made applicable to प्रसंस्तीतः &c.