tamer: asg::expr::ExprOp: Future implementation note

I had wanted to implement expression operations in terms of user-defined
functions (where primitives are just marked as intrinsic), and would still
like to, but I need to get this thing working, so I'll just include a note
for now.

Yes, TAMER's formalisms are inspired by APL, if that hasn't been documented
anywhere yet.

DEV-13160

tamer/src/asg/expr.rs

@@ -66,6 +66,10 @@ impl Display for Expr {
 }
 
 /// Expression operation type.
+///
+/// TODO: Ideally this will be replaced with arbitrary binary (dyadic)
+///   functions defined within the language of TAME itself,
+///     as was the original plan with TAMER.
 #[derive(Debug, PartialEq, Eq)]
 pub enum ExprOp {
     Sum,