Let Γ(X, ~, t, I) be an incidence geometry and let J be a subset of I. Then the J--truncation of Γ is the geometry whose set of elements is t - 1(J), together with the restricted type function and incidence relation.
Let J ⊆I. The J--truncation of the coset geometry Γ(G;(Gi)i ∈I) is the coset geometry Γ( G; (Gj)j∈J).
Given an incidence geometry D and t a subset of the set of types of D, return the t--truncation of D as an incidence geometry.
Given a coset geometry C and t a subset of the set of types of C, return the t--truncation of C as a coset geometry.