The augment iṭ added to valādi ārdhadhātuka affixes, becomes lengthened, except in the personal endings of the liṭ (Perfect Tense), after the root graha,
As ग्रहीता, ग्रहीतुम्, ग्रहीतव्यम् ॥ Why not in the Perfect? Observe जगृहिव, जगृहिम् ॥ The lengthening takes place of the इट् taught in 7.2.35, and does not refer to the चिण्वद् इट् of 6.4.62 : as ग्राहिता, ग्रहिष्यते ॥,