Add Shift+drop to attach node without creating enhanced dependency

Fix stylesheet to display nodes without deps in cyan.
Add more details to the documentation.

README.pod

@@ -6,9 +6,32 @@ ud - TrEd extension to display and edit Universal Dependencies
 
 =head1 DESCRIPTION
 
-Use mouse to change the tree structure. Drop a node with C<Ctrl> to add or
-remove an enhanced dependency. Press C<d> to edit the current node's
-C<deprel>.
+=over 2
+
+=item *
+
+Dropping node C<a> on node C<b> attaches node C<a> to node C<b>. An enhanced
+dependency is created together with the basic one, if an enhanced dependency
+existed to the old or new parent, it is removed.
+
+=item *
+
+Dropping node C<a> on node C<b> while pressing C<Control> toggles existence of
+an enhanced dependency of node C<a> on node C<b>. If a dependency was created,
+the user is asked for its type. This works even when C<a> already is C<b>'s
+parent.
+
+=item *
+
+Dropping node C<a> on node C<b> while pressing C<Shift> attaches node C<a> to
+node C<b> without creating an enhanced dependency. If an enhanced dependency
+existed to the old or new parent, it is removed.
+
+=item *
+
+Pressing C<d> opens a dialog box to edit the current node's dependency type.
+If previously an enhanced dependency of the same type as the basic dependency
+existed, it is changed together with it, otherwise, it stays unchanged.
 
 =head1 INSTALLATION
 

ud/contrib/ud/ud.inc

@@ -142,13 +142,16 @@
         } else {
             return if $target == $parent;
 
-            my @deps = (_create_container(
-                            $target->{ord}, $node->{deprel}),
-                        grep $_->{func} ne $node->{deprel}
-                             && $_->content != $parent->{ord},
+            my $with_enhanced = ! (defined $mod && $mod eq 'Shift');
+            my @deps = ($with_enhanced
+                            ? _create_container($target->{ord},
+                                                $node->{deprel})
+                            : (),
+                        grep $_->content != $parent->{ord}
+                             && $_->content != $target->{ord},
                         ListV($node->{deps}));
             delete $node->{deps};
-            $node->{deps} = List(@deps);
+            $node->{deps} = List(@deps) if @deps;
             CutPaste($node, $target);
             ChangingFile(1);
         }

ud/stylesheets/UD

@@ -6,32 +6,33 @@ node: ${id}<? '#{grey}' if $${ord} =~ /\./ ?>${form}<? $${lemma} && $${lemma} ne
 node: #{darkcyan}${upostag}<? $${id} ? "" : "." ?>#{blue}${deprel}
 
 style: <?
-    if ($this->{deps}) {
-        my @deps = grep {
-            ($_->content // 0) ne ($this->parent->{ord} // 0)
-       } ListV($this->{deps});
-
-       my $is_parent_in_deps = grep $_->content == $this->parent->{ord}
-                                    && $_->{func} eq $this->{deprel},
-                               ListV($this->{deps});
-
-        my $coords = join '&', 'n,n,p,p', map {
-            my ($parent, $func) = ($_->content, $_->{func});
-            my $expression = $parent ? "[ord=$parent]" : '[! $this->root !]';
-            "n,n,(x$expression+xn)/2,(y$expression+yn)/2.5,x$expression,y$expression"
-        } @deps;
-
-        my $arrows = join '&', 'none', ('last') x @deps;
-        my $colours = join '&', ($is_parent_in_deps ? 'grey' : 'cyan'), ('orange') x @deps;
-        my $widths = join '&', ($is_parent_in_deps ? '2' : '1'), ('1') x @deps;
-        my $smooth = join '&', '0', ('1') x @deps;
-        my $hints = join '&', "", map "$_->{func}", @deps;
-        my $deco = join '&', "", map "shape=text;font=SansSerif 9;fill=magenta;coords=n,n;start=20;text=$_->{func}", @deps;
-
-        "#{Line-coords:$coords}#{Line-fill:$colours}#{Line-arrow:$arrows}"
-        . "#{Line-width:$widths}#{Line-smooth:$smooth}#{Line-hint:$hints}"
-        . "#{Line-decoration:$deco}"
-        . ('#{Oval-fill:whitesmoke}#{CurrentOval-fill:pink}#{Oval-outline:cyan}' x $this->{ord} =~ /\./);
-    }
+    my @deps = grep {
+        ($_->content // 0) ne ($this->parent->{ord} // 0)
+    } ListV($this->{deps});
+
+    my $is_parent_in_deps = grep $_->content == $this->parent->{ord}
+                                 && $_->{func} eq $this->{deprel},
+                            ListV($this->{deps});
+    warn $this->{ord}, ' ', $is_parent_in_deps ? 'yes' : 'no';
+
+    my $coords = join '&', 'n,n,p,p', map {
+        my ($parent, $func) = ($_->content, $_->{func});
+        my $expression = $parent ? "[ord=$parent]" : '[! $this->root !]';
+        "n,n,(x$expression+xn)/2,(y$expression+yn)/2.5,x$expression,y$expression"
+    } @deps;
+
+    my $arrows = join '&', 'none', ('last') x @deps;
+    my $colours = join '&', ($is_parent_in_deps ? 'grey' : 'cyan'), ('orange') x @deps;
+    my $widths = join '&', ($is_parent_in_deps ? '2' : '1'), ('1') x @deps;
+    my $smooth = join '&', '0', ('1') x @deps;
+    my $hints = join '&', "", map "$_->{func}", @deps;
+    my $deco = join '&', "", map "shape=text;font=SansSerif 9;fill=magenta;coords=n,n;start=20;text=$_->{func}", @deps;
+
+    warn "\t$colours | $widths";
+
+    "#{Line-coords:$coords}#{Line-fill:$colours}#{Line-arrow:$arrows}"
+    . "#{Line-width:$widths}#{Line-smooth:$smooth}#{Line-hint:$hints}"
+    . "#{Line-decoration:$deco}"
+    . ('#{Oval-fill:whitesmoke}#{CurrentOval-fill:pink}#{Oval-outline:cyan}' x $this->{ord} =~ /\./);
 
 ?>