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