Dear list members,
A long time ago (January 2015) there was a fascinating discussion about metric anomolies  in the vedas.  The  Aitareya Brāhmana (na vā ekenākṣareṇa chandāṃsi viyanti na dvābhyām)  and the  Rk-prātiśākhya were mentioned in the discussion but no-one mentioned Piṅgala .

But it seems  Piṅgala  wrote about this.  By chance while looking for something else I came across Pingalacchandahsutra: A Study by Asoke Chatterjee Sastri.  In case this is of interest to the participants in the discussion and others, the link is https://archive.org/download/dli.calcutta.11212/cu_pub596.pdf

Some selections from the book:

 Pages vii -viii

The Vedic metres at the same time do never lose its impress pattern on excess or loss of one

or two syllables. This overstepping or shortness of due limits does not stand in the way of their

fullfilment of the definition of the particular metre. The Aitereya Brāhmaṇa claims that by one

letter or by two letters the metres are not disqualified (na vā ekena akṣareṇа chandāṁsi viyanti

na dvābhyām), Where the number of the syllable is less, the euphonic combination should be

disjoined and Pingala has rightly framed a rule in this regard (Piṅgala—3-2).


pages 84-85

. . .In the second sūtra of this chapter [three] the author has prescribed the rules regarding filling of foot, in gāyatri etc. where syllables fall short of 

the required number, there we fill the pada (foot) with iy, uv etc. As for example ‘tat savitur vareṇyam’ is cited by the vṛttikāra. 

Here as there are seven syllables only by the side of other two feet of eight syllables each, one should fill the foot adding one more syllable, i.e., iy. 

Then the form will be 'tat savitur vareṇiyam'. . .


page 92

If the characteristic features of the metres as shown by Piṅgala fail to hold good to all the Vedic

metres, all these may be explained with the help of nicṛt, bhūrik, virāṭ etc. . . .


page 61

...

The metre here is triṣṭup. The first foot of the mantra consists of ten syllables, second foot

of eleven syllables, third of nine syllables and fourth foot of ten syllables. . . . scholars took

resort to and devised 'iyādipūraṇa'. By this rule if we add one syllable in the first foot, two in

the second foot and again one in the third foot then it will be triṣṭup metre.


page 89

. . . There are two other kinds of metres named nicṛt and bhūrik. Generally gāyatrī consists of 

twentyfour syllables. But when we find one syllable less, it is called nicṛt gāyatrī and when it

 
contains one syllable in excess it is known by the name bhūrik gāyatrī. Other metres are also to

be divided thus. Similarly when two syllables less or more are detected in a gāyatrī, they are virāṭ gāyatrī and svarāṭ gāyatrī respectively.

 Other metres also follow the same principle. If there exists

 any doubt in the determination of metre, the first foot is regarded as the deciding factor.


Harry Spier