make op/3 a predicate call from the query context

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "scryer-prolog"
-version = "0.8.28"
+version = "0.8.29"
 authors = ["Mark Thom <markjordanthom@gmail.com>"]
 repository = "https://github.com/mthom/scryer-prolog"
 description = "A modern Prolog implementation written mostly in Rust."

src/prolog/machine/toplevel.rs

@@ -693,6 +693,7 @@ impl RelationWorker {
                     Ok(TopLevel::Declaration(Declaration::Hook(hook, clause, queue)))
                 } else if name.as_str() == "?-" {
                     match setup_declaration(terms.iter().cloned().collect()) {
+                        Ok(Declaration::Op(..)) => {}, // this is now a predicate call in the query context.
                         Ok(decl) => return Ok(TopLevel::Declaration(decl)),
                         _ => {}
                     };