In the roots sphur and sphul 'to move', there is the substitution of the ā for the diphthong when the affix ghañ follows.,
Thus विस्फारः instead of विस्फोरः, and विस्फालः instead of विस्फोलः ॥ By 8.3.76, the स is optionally changed to ष after वि, as विष्फारः, and विष्फालः ॥,