  This is a first of our papers devoted to "noncommutative topology and graph
theory". Its origin is the paper math.QA/0002238 by I. Gelfand, V. Retakh, and
R.L. Wilson where a new class of noncommutative algebras $Q_n$ was introduced.
The algebra $Q_n$ is closely related to factorizations of a generic polynomial
of degree $n$ over a division algebra into linear factors.
