hoon, jets: add +cap

hoon/scaffolding/playpen.hoon

@@ -133,6 +133,20 @@
   ?:  =(0 b)  a
   $(a (dec a), b (dec b))
 ::
+::  Tree addressing
+::
+++  cap                                                 ::  index in head or tail
+  ~/  %cap                                              
+  |=  a=@
+  ~>  %sham.%cap
+  ^-  ?(%2 %3)
+  ?-  a
+    %2        %2
+    %3        %3
+    ?(%0 %1)  !!
+    *         $(a (div a 2))
+  ==
+::
 ::  List logic
 ::
 ++  flop                                                ::  reverse

rust/ares/src/jets.rs

@@ -1,8 +1,10 @@
 pub mod math;
 pub mod mink;
+pub mod tree;
 
 use crate::jets::math::*;
 use crate::jets::mink::*;
+use crate::jets::tree::*;
 use crate::mem::NockStack;
 use crate::newt::Newt;
 use crate::noun::{self, Noun, Slots};
@@ -66,6 +68,9 @@ pub fn get_jet(jet_name: Noun) -> Option<Jet> {
         tas!(b"met") => Some(jet_met),
         tas!(b"mug") => Some(jet_mug),
         tas!(b"rev") => Some(jet_rev),
+        //
+        tas!(b"cap") => Some(jet_cap),
+        //
         tas!(b"mink") => Some(jet_mink),
         _ => {
             // eprintln!("Unknown jet: {:?}", jet_name);