Reference:
http://arxiv.org/abs/0711.3503
Squangles
This is the source page for squangles, which are Markov invariants
as decribed in Sumner et al., available on the arXiv (reference
above). Documentation: pdf, html
R source code here.
Polynomials sqg1, sqg2, sqg3.
Stangle
The stangle is another Markov invariant applicable to triplets.
The stangle polynomial is here (see documentation for squangles for how to
interpret this file).