Add new reducewith sub intended to be the backend for [op] (though th…

…at is not wired up yet).

src/core/metaops.pm

@@ -96,6 +96,27 @@ our multi sub hyper(&op, $arg) {
     hyper(&op, $arg.list)
 }
 
+our multi sub reducewith(&op, Iterable $an-iterable, :$chaining, :$right-assoc) {
+    my $ai = $an-iterable.iterator;
+    $ai = $ai.Seq.reverse.iterator if $right-assoc;
+
+    my $result = $ai.get;
+    if $result ~~ EMPTY {
+        return &op();
+    }
+
+    loop {
+        my $next = $ai.get;
+        last if $next ~~ EMPTY;
+        if ($right-assoc) {
+            $result = &op($next, $result);
+        }
+        else {
+            $result = &op($result, $next);
+        }
+    }
+    $result;
+}
 
 # degenerate case of operators, to be used by reduce() for the 0-ary case
 # this fails for operators defined in PIR, so some of them are commented out.