  Simplicial homology manifolds are proposed as an interesting class of
geometric objects, more general than topological manifolds but still quite
tractable, in which questions about the microstructure of space-time can be
naturally formulated. Their string orientations are classified by $H^3$ with
coefficients in an extension of the usual group of D-brane charges, by
cobordism classes of homology three-spheres with trivial Rokhlin invariant.
