  Given an associative, not necessarily commutative, ring R with identity, a
formal matrix calculus is introduced and developed for pairs of matrices over
R. This calculus subsumes the theory of homogeneous systems of linear equations
with coefficients in R. In the case when the ring R is a field, every pair is
equivalent to a homogeneous system.
  Using the formal matrix calculus, two alternate presentations are given for
the Grothendieck group $K_0 (R-mod, \oplus)$ of the category R-mod of finitely
presented modules. One of these presentations suggests a homological
interpretation, and so a complex is introduced whose 0-dimensional homology is
naturally isomorphic to $K_0 (R-mod, \oplus).$ A computation shows that if R =
k is a field, then the 1-dimensional homology group is given by the
abelianization of the multiplicative group of k, modulo the subgroup {1, -1}.
  The formal matrix calculus, which consists of three rules of matrix
operation, is the syntax of a deductive system whose completeness was proved by
Prest. The three rules of inference of this deductive system correspond to the
three rules of matrix operation, which appear in the formal matrix calculus as
the Rules of Divisibility.
