possible fix for multi-method import bug

This patch allows you to import multi methods from several modules.
It merges the dispachtee lists. Two identical dispatchee signatures
dont clash yet, since I am unable to compare signatures.

src/Perl6/World.pm

@@ -264,14 +264,67 @@ class Perl6::World is HLL::World {
         my %to_install;
         my @clash;
         for %stash {
-            if $target.symbol($_.key) {
-                # XXX TODO: Merge handling.
-                nqp::push(@clash, $_.key);
+            my $foreign_proto; # of the package we import
+            my $installed_proto; # of the current lexpad (target)
+
+            try {
+                $foreign_proto := self.find_symbol([$_.key]);
+            }
+
+            # It is a multi, so we need to install a proto in the lexpad and add dispatchees.
+            # Note: NQPCursorRole does not support get_bool, therefor skipped.
+            if $_.key ne 'NQPCursorRole' && $foreign_proto && $foreign_proto.is_dispatcher {
+                if $_.value && $_.value.is_dispatcher {
+                    $foreign_proto := $_.value;
+                }
+
+                my @installed_dispatchees;
+
+                # Symbol is known, we need to add dispatchees to the proto that is already there.
+                if $target.symbol($_.key) {
+                    # reuse the already installed proto
+                    $installed_proto := $target.symbol($_.key)<value>;
+
+                    # remember the installed dispatchees so that we dont add duplicates
+                    for $installed_proto.dispatchees -> $dispatchee {
+                        @installed_dispatchees.push: $dispatchee;
+                    }
+                }
+                # Symbol not yet known, we should install a proto here.
+                else {
+                    # we will add dispatchees to this later
+                    $installed_proto := self.derive_dispatcher( $foreign_proto );
+
+                    # install the foreign symbol (proto?) into our lexpad
+                    self.install_lexical_symbol($target, $_.key, $_.value, :clone(1));
+                }
+
+                if $foreign_proto.dispatchees {
+                    for $foreign_proto.dispatchees -> $foreign_dispatchee {
+                        my $found_new := 0;
+                        for @installed_dispatchees -> $installed_dispatchee {
+                            if $installed_dispatchee =:= $foreign_dispatchee {
+                                $found_new := 1;
+                                last;
+                            }
+                        }
+
+                        unless $found_new {
+                            self.add_dispatchee_to_proto($installed_proto, $foreign_dispatchee);
+                        }
+                    }
+                }
             }
             else {
-                $target.symbol($_.key, :scope('lexical'), :value($_.value));
-                $target[0].push(QAST::Var.new( :scope('lexical'), :name($_.key), :decl('var') ));
-                %to_install{$_.key} := $_.value;
+                if $target.symbol($_.key) {
+                    # If it is not a multi and the symbol is already there, it is a redeclaration.
+                    nqp::push(@clash, $_.key);
+                }
+                else {
+                    $target.symbol($_.key, :scope('lexical'), :value($_.value));
+                    $target[0].push(QAST::Var.new( :scope('lexical'), :name($_.key), :decl('var') ));
+                    %to_install{$_.key} := $_.value;
+                }
             }
         }
         if +@clash {