The final s of tās (the character of the second future) and as 'to be' is elided before an affix beginning with r,
As कर्तारौ, कर्त्तारः, अध्येतारौ, अध्येतारः ॥ The स् of अस् 'to be' is elided in the Perfect, because it is there that a र् beginning affix can follow it. Thus व्यतिरे (See 6.4.111. According to the reading of Padamanjari, there can be no example of अस् ॥ अस्तेरुदाहरणं न प्रदर्शितं, रेफादेरसम्भवात् ॥,