Adapted from Chapter 5 of 'Saṅgītaratnākara - A Study' by R.Rangaramanuja Ayyangar, 2nd Ed.
Naṣṭam (that which is lost, hidden) was a method to determine the rūpam (form) of a specified step in a prastāra. Both Naṣṭam and
uddiṣṭam methods used saṅkhyā series. The number of permutation steps that ended in each of the four tāla elements and their relative positioning in
the series assumed significance.
The naṣṭam method is illustrated through an example to determine the tāla rūpam for the seventh step in the laghu-pluta prastāra.
Note: The derivation process is interspersed with a tabular interpretation represented through steps 'a' to 'm'.
1. Write down the saṅkhyā series for this prastāra (step a in table below). The series for the laghu-pluta prastāra extends till the
value 60. This value represents the total number of permutation steps in the prastāra. The number of druta, laghu, guru and pluta ending permutation
steps are at one, two, four and six blocks to the left of this position. Tag these positions with a set of five markers, say, m0, m1, m2, m3 and m4 (step b).
2. At position m0, deduct the desired step number from the total number of permutation steps in the prastāra and obtain the remainder.
60 - 7 = 53 (step c). Only this first reduction is carried out at position m0. All subsequent reductions are at positions starting from m1 onwards.