First cut of nested lexical multis, which derive a dispatcher based o…

…n the one in the outer lexical scope.

src/Perl6/Actions.pm

@@ -1138,16 +1138,21 @@ class Perl6::Actions is HLL::Actions {
                 }
                 else {
                     # None; search outer scopes.
+                    my $new_proto;
                     try {
                         $proto := $*ST.find_symbol([$name]);
                     }
                     if $proto {
                         # Found in outer scope. Need to derive.
-                        $/.CURSOR.panic("proto derivation NYI");
+                        $new_proto := $*ST.derive_dispatcher($proto);
                     }
                     else {
                         $/.CURSOR.panic("proto auto-gen NYI");
                     }
+
+                    # Install in current scope.
+                    $*ST.install_lexical_symbol($outer, $name, $new_proto);
+                    $proto := $new_proto;
                 }
 
                 # Ensure it's actually a dispatcher.

src/Perl6/SymbolTable.pm

@@ -626,6 +626,23 @@ class Perl6::SymbolTable is HLL::Compiler::SerializationContextBuilder {
         )));
     }
 
+    # Derives a proto to get a dispatch.
+    method derive_dispatcher($proto) {
+        # Immediately do so and add to SC.
+        my $derived := $proto.derive_dispatcher();
+        my $slot    := self.add_object($derived);
+
+        # Add deserialization action.
+        my $des := PAST::Op.new(
+            :pasttype('callmethod'), :name('derive_dispatcher'),
+            self.get_object_sc_ref_past($proto));
+        self.add_event(:deserialize_past(
+            self.set_slot_past($slot, self.set_cur_sc($des))
+        ));
+
+        return $derived;
+    }
+
     # Creates a new container descriptor and adds it to the SC.
     method create_container_descriptor($of, $rw, $name) {
         # Create descriptor object now.