  We prove that the coindex of the box complex $\mathrm{B}(H)$ of a graph $H$
can be measured by the generalised Mycielski graphs which admit a homomorphism
to it. As a consequence, we exhibit for every graph $H$ a system of linear
equations solvable in polynomial time, with the following properties: If the
system has no solutions, then $\mathrm{coind}(\mathrm{B}(H)) + 2 \leq 3$; if
the system has solutions, then $\chi(H) \geq 4$. We generalise the method to
other bounds on chromatic numbers using linear algebra.
