Add documentation of ClipBic

Refer to Arxiv paper and mention bicyclic semigroups.

crates/encoding/src/clip.rs

diff --git a/crates/encoding/src/clip.rs b/crates/encoding/src/clip.rs
index 025b86b..1a68fba 100644
--- a/crates/encoding/src/clip.rs
+++ b/crates/encoding/src/clip.rs
@@ -4,10 +4,18 @@
 use bytemuck::{Pod, Zeroable};
 
 /// Clip stack element.
+///
+/// This is the bicyclic semigroup, a monoid useful for representing
+/// stack depth. There is considerably more detail in the draft paper
+///  [Fast GPU bounding boxes on tree-structured scenes].
+///
+/// [Fast GPU bounding boxes on tree-structured scenes]: https://arxiv.org/abs/2205.11659
 #[derive(Copy, Clone, Pod, Zeroable, Debug, Default)]
 #[repr(C)]
 pub struct ClipBic {
+    /// When interpreted as a stack operation, the number of pop operations.
     pub a: u32,
+    /// When interpreted as a stack operation, the number of push operations.
     pub b: u32,
 }
 
@@ -47,6 +55,12 @@
         ClipBic { a, b }
     }
 
+    /// The bicyclic semigroup operation.
+    ///
+    /// This operation is associative. When interpreted as a stack
+    /// operation, it represents doing the pops of `self`, the pushes of
+    /// `self`, the pops of `other`, and the pushes of `other`. The middle
+    /// two can cancel each other out.
     pub fn combine(self, other: ClipBic) -> Self {
         let m = self.b.min(other.a);
         ClipBic::new(self.a + other.a - m, self.b + other.b - m)