cross: list: add some conveniences to query list length and access the first element

crates/cross/src/compound/list/mod.rs

@@ -1,4 +1,4 @@
-use crate::compound::peano::Peano;
+use crate::compound::peano::{Peano, P0};
 
 mod flat;
 // mod linked;
@@ -47,6 +47,32 @@ pub trait IndexableExplicit {
     {
         Indexable::set(self, v)
     }
+
+    fn get_first(&self) -> <Self as Indexable<P0>>::Element
+    where
+        Self: Indexable<P0>,
+        Self::Element: Copy,
+    {
+        Indexable::get(self)
+    }
+    fn get_first_ref(&self) -> &<Self as Indexable<P0>>::Element
+    where
+        Self: Indexable<P0>,
+    {
+        Indexable::get_ref(self)
+    }
+    fn get_first_mut(&mut self) -> &mut <Self as Indexable<P0>>::Element
+    where
+        Self: Indexable<P0>,
+    {
+        Indexable::get_mut(self)
+    }
+    fn set_first(&mut self, v: Self::Element)
+    where
+        Self: Indexable<P0>,
+    {
+        Indexable::set(self, v)
+    }
 }
 
 impl<L> IndexableExplicit for L {}
@@ -57,6 +83,9 @@ pub type ElementAt<P, L> = <L as Indexable<P>>::Element;
 /// implemented by any List (including the Null, empty list)
 pub trait Meta {
     type Length: Peano;
+    fn len(&self) -> u32 {
+        Self::Length::VALUE
+    }
 }