Implement addition for OwnedLiterals.

src/renderer/context_element.rs

@@ -4,6 +4,7 @@ use crate::renderer::errors::RenderError;
 use crate::renderer::errors::WalkError;
 use crate::renderer::DustRenderer;
 use std::any::Any;
+use std::ops::Add;
 use std::rc::Rc;
 use std::{cmp::Ordering, fmt::Debug};
 
@@ -61,6 +62,8 @@ pub trait CompareContextElement: CastToAny {
     fn equals(&self, other: &dyn ContextElement) -> bool;
 
     fn partial_compare(&self, other: &dyn ContextElement) -> Option<Ordering>;
+
+    fn math_add<'a>(&self, other: &dyn ContextElement) -> Option<IceResult<'a>>;
 }
 
 impl<C: 'static + ContextElement> CastToAny for C {
@@ -81,6 +84,14 @@ impl<'a, 'b> PartialOrd<&'b dyn ContextElement> for &'a dyn ContextElement {
     }
 }
 
+impl<'a> Add<&'a dyn ContextElement> for &'a dyn ContextElement {
+    type Output = Option<IceResult<'a>>;
+
+    fn add(self, other: &'a dyn ContextElement) -> Self::Output {
+        self.math_add(other)
+    }
+}
+
 pub trait FromContextElement {
     fn from_context_element(&self) -> &dyn IntoContextElement;
 }
@@ -91,7 +102,7 @@ impl<C: ContextElement> FromContextElement for C {
     }
 }
 
-pub trait IntoContextElement: Debug + Walkable /* + CloneIntoBoxedContextElement*/ {
+pub trait IntoContextElement: Debug + Walkable {
     fn into_context_element<'a>(
         &'a self,
         renderer: &DustRenderer,