hidden: Document the convention about last face

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

diff --git a/src/gamestate.rs b/src/gamestate.rs
index d50baaed20a9732ce8b9491b7e4550110feaecdf..1a308d52a65430d6ba302a32089873585cdcf155 100644 (file)
--- a/src/gamestate.rs
+++ b/src/gamestate.rs
@@ -122,6 +122,7 @@ pub struct UoDescription {
 
 #[typetag::serde]
 pub trait Piece: Outline + Send + Debug {
+  /// by convention, occult face is nfaces-1
   fn nfaces(&self) -> RawFaceId;
 
   #[throws(InternalError)]