# Shadows

Let $\Gamma(X,\sim,t,I)$ be an incidence geometry. Let $J$ be a subset of $I$ and let $F$ be a flag of $\Gamma$ such that $t(F) \cap J = \emptyset$. The $J$*-shadow* of the flag $F$, denoted by $\sigma_J(F)$, is the set of flags of type $J$ in the residue of the flag $F$.

## `Shadow(D, I, F): IncGeom, Set, Set -> SetIndx`

Given an incidence geometry $D$, a subset $I$ of the set of types of $D$ and a flag $F$ of $D$, return the $I$-shadow of the flag $F$ as an indexed set of subsets of points of $D$.