  For a commutative ring $A$ we consider a related graph, $\Gamma(A)$, whose
vertices are the unimodular rows of length $2$ up to multiplication by units.
We prove that $\Gamma(A)$ is path-connected if and only if $A$ is a
$\mathrm{GE}_2$-ring, in the terminology of P. M. Cohn. Furthermore, if $Y(A)$
denotes the clique complex of $\Gamma(A)$, we prove that $Y(A)$ is simply
connected if and only if $A$ is universal for $\mathrm{GE}_2$. More precisely,
our main theorem is that for any commutative ring $A$ the fundamental group of
$Y(A)$ is isomorphic to the group $K_2(2,A)$ modulo the subgroup generated by
symbols.
