  Canonical differential equations matrix is the 1-$\mathrm{d}\log$-form
coefficients of projecting ($n$+1)-$\mathrm{d}\log$-forms onto
$n$-$\mathrm{d}\log$-forms master integrand. Intersection number with dual form
in relative cohomology could achieve the projecting and give selection rules of
canonical differential equations from the pole structure of $\mathrm{d}\log$
master integrand.
