This page is dedicated to the paper 4-tangrams are 4-avoidable by Pascal Ochem and Theo Pierron.

ch_rep.c checks that the h-image of every 6-ary (6/5+)-free word of length at most lim is (5/4+, 9)-free.
ch_tan.c checks that for every distinct letters a and b, h(ab) contains no occurrence of a pattern in S_4 of length at most l.
bound.c computes the upper bound on an occurrence in a (5/4+, 9)-free word for every pattern in S_4.

Usage:
./ch_rep lim
./ch_tan l
./bound

To compile:
gcc -O3 -o ch_rep ch_rep.c
gcc -O3 -o ch_tan ch_tan.c
gcc -O3 -o bound bound.c