Abstract
We provide a basic formal framework for the theory of chamber
complexes and Coxeter systems, and for buildings as thick chamber
complexes endowed with a system of apartments. Along the way, we
develop some of the general theory of abstract simplicial complexes
and of groups (relying on the group_add class for the basics),
including free groups and group presentations, and their universal
properties. The main results verified are that the deletion condition
is both necessary and sufficient for a group with a set of generators
of order two to be a Coxeter system, and that the apartments in a
(thick) building are all uniformly Coxeter.