The s of tās (the character of the second future) and as 'to be' is substituted h before the personal-ending e,
As कर्त्ताहे, and व्यति हे from अस् ॥ See 6.4.111. The ह substitution does not take place before the Personal ending एश् 3.4.8, as एधामासे ॥ The reason of this is that the ए referred to in this sutra, is that ए which can come after तास्; and that very ए should also come after अस् ॥ The ए that can come after तास् is the Ist Person singular ए; and not this ए of the Perfect 3rd Person singular. Others say, this even sarvadhatuka, and therefore not here सुप्तोऽहं त्वां प्रेक्षामासे ॥,