add compare predicate.

README.md

@@ -23,7 +23,7 @@ Extend rusty-wam to include the following, among other features:
 * Built-in control operators (`,`, `;`, `->`, etc.) (_done_).
 * Built-in predicates for list processing and top-level declarative
   control (`setup_call_control/3`, `call_with_inference_limit/3`,
-  etc.) (_in progress_).
+  etc.) (_done_).
 * Add a rudimentary module system.
 * Attributed variables using the SICStus Prolog interface and
   semantics. Adding coroutines like `dif/2`, `freeze/2`, etc.
@@ -98,6 +98,7 @@ The following predicates are built-in to rusty-wam.
 * `call/1..63`
 * `call_with_inference_limit/3`
 * `catch/3`
+* `compare/3`
 * `compound/1`
 * `display/1`
 * `duplicate_term/2`