libmetro

Polyrhythm is the original motivator of libmetro. Polyrhythm as defined by britannica.com is:
>Polyrhythm, also called Crossrhythm, the simultaneous combination of contrasting rhythms in a musical composition. Rhythmic conflicts, or crossrhythms, may occur within a single metre (e.g., two eighth notes against triplet eighths) or may be reinforced by simultaneous combinations of conflicting metres.
I first learned of polyrhythm through metal bands (Meshuggah, Animals as Leaders, Periphery, etc.):
Polyrhythm features in African music (and may have even originated there):
Polyrhythms are expressed rather simply in libmetro. They need to be manually constructed in a single measure using the least common multiple of both rhythms, which is a familiar exercise to anyone studying polyrythms or following basic tutorials (as I did  linked right below).
Resources, tutorials:
As before, the metronome files below can be viewed here.
To create a 3:2, we need to find the LCM of 3 and 2, which is 6, spread the triple at indices 0,2,4, and the duple at indices 0,3:
poly_32.txt, file format 1:
This looks better in file format 2, in poly_32_format2.txt:
You'll see see I added sine wave beeps/clicks to make clear how the triples and duples are overlaid on 6 beats:
3:2, 250bpm:
2:3 (inverted timbres of 3:2), 250bpm:
There's a fullyfledged pureC++ 4:3 example. where one can specify how many measures of 4, 3, 4:3, and only clicks (no beats) to play.
The idea is that you could practice X measures of 4, Y measures of 3, Z measures of 4:3, and W measures of only clicks (on which you're expected to correctly play the 4:3), repeated over and over. This can perhaps be of value to drummers.
Note that this concept is expressible with the text file method, but requires one to copy/paste blocks of measures (and change the indices manually), which is tedious.
Test run with parameters 2 2 2 2 300, i.e. 2 measures of 4, 2 measures of 3, 2 measures of 4:3, and 2 "blank" measures of just clicks, at 300bpm:
poly_53.txt, file format 1:
poly_53_format2.txt:
5:3, 300bpm:
A suggestion from Victor of a more complicated polyrhythm  named '35_swing' because it can be thought of as '3/5, 5 as a swing pattern':
poly_35_swing_format2.txt:
At 394bpm: