Start to answer exercise 13

This first answer applies a condition too strong for the requirements:
both sets related by transposition and inversion get removed, and we
obtain the 12 sets from 3-1 to 3-12 listed above the exercise in the book.
The condition should be relaxed to keep sets related by inversion.
For example, the set (0,4,6) is related to (0,2.6) (mirrored),
but should be preserved when only transpositions are considered.
Complete exercise 13, using lexical order to order and filter sets

Each chord can be transposed to three different sets (all identical
in the case of chord (0,4,8)) starting with note 0:

* (0,a,b)
* (0,b-a,12-a)
* (0,12-b,12+a-b)

Comparing these three sets using lexical order allows to keep
only the minimum set, skipping the two variations obtained
by transposition.

There are 55 sets starting with 0 among the 220 sets of three notes.
If you set aside (0,4,8) which is identical to itself in each of
the tree transpositions described above, only one in three sets
is kept in the remaining 54 sets, which results in 1 + 54/3 = 19 sets.