Shadow Spaces

Let Gamma(X, ~, t, I) be an incidence geometry. Let J be a subset of I. The shadow space Gamma(J) of Gamma is the incidence structure whose point-set is the set of flags of type J of Gamma and whose blocks are the J - shadows of the flags of Gamma.

ShadowSpace(D,I) : IncGeom, Set -> Inc

Given an incidence geometry D and a subset I of the set of types of D, return the shadow space D(I) as an incidence structure.