document Proxy

WANTED

@@ -19,7 +19,6 @@ Synatax features:
 
 API docs:
     * KeyReducer
-    * Proxy
 
 Builtins:
     * Str.substr-eq

lib/Type/Proxy.pod

@@ -0,0 +1,37 @@
+=begin pod
+
+=TITLE class Proxy
+
+=SUBTITLE Container with custom storage and retrieval
+
+    class Proxy { ... }
+
+A Proxy is an object that allows you to execute whenever a value is retrieved
+from a contaner (C<FETCH>) or when it is set (C<STORE>).
+
+To create a container that returns twice of what was stored in it, you do
+something like this:
+
+   sub double() is rw {
+       my $storage = 0;
+       Proxy.new(
+           FETCH => method ()     { $storage },
+           STORE => method ($new) { $storage = 2 * $new }
+       )
+    }
+    my $doubled := double();
+    $doubled = 4;
+    say $doubled;       # 8
+
+=head1 Methods
+
+=head2 method new
+
+    method new(:&FETCH!, :&STORE!) returns Proxy:D
+
+Creates a new C<Proxy> object. C<&FETCH> is called with one argument (the
+proxy object) when the value is accessed, and must return the value that the
+fetch produces. C<&STORE> is called with two arguments (the proxy object, and
+the new value) when a new value is stored in the container.
+
+=end pod