diff --git a/src/model.rs b/src/model.rs
index ac69cc7..f94f823 100644
--- a/src/model.rs
+++ b/src/model.rs
@@ -73,7 +73,7 @@ impl VertexId{
 	}
 }
 pub struct IndexedVertexList{
-	pub vertices:Vec<VertexId>,
+	vertices:Vec<VertexId>,
 }
 #[derive(Clone,Copy,Hash,PartialEq,Eq)]
 pub struct PolygonGroupId(u32);
@@ -93,6 +93,11 @@ impl PolygonGroup{
 			//PolygonGroup::TriangleStrip(strip)=>return strip.windows(3).enumerate().map(|(i,s)|if i&0!=0{return s.iter().rev()}else{return s.iter()}),
 		}
 	}
+	pub fn map_vertex_id<F:Fn(VertexId)->VertexId>(self,f:F)->Self{
+		match self{
+			PolygonGroup::PolygonList(polys)=>Self::PolygonList(polys.into_iter().map(|ivl|IndexedVertexList{vertices:ivl.vertices.into_iter().map(&f).collect()}).collect()),
+		}
+	}
 }
 /// Ah yes, a group of things to render at the same time
 #[derive(Clone,Copy,Hash,Eq,PartialEq)]