Adds uninstantiable with examples.

This closes #1863. It advances #1957, but still some information on
how to use traits in varibles and attributes is going to be needed.

Author: JJ

doc/Language/traits.pod6

@@ -44,6 +44,26 @@ The L<meta object protocol|/language/mop> uses it by default for every object
 and class unless specified otherwise; for that reason, it is in general not
 necessary unless you are effectively working with that interface.
 
+The X<Uninstantiable representation trait> is not so much related to the
+representation as related to what can be done with an specific class; it
+effective prevents the creation of instances of the class via C<new>
+
+=begin code
+constant @IMM = <Innie Minnie Moe>;
+
+class don't-instantiate is repr('Uninstantiable') {
+    my $.counter;
+
+    method imm () {
+        return @IMM[ $.counter++ mod @IMM.elems ];
+    }
+}
+say don't-instantiate.imm for ^10;
+=end code
+
+Uninstantiable classes can still be used via their class variables and methods, as above. However, trying to instantiate them this way: C<my $do-instantiate = don't-instantiate.new;> will yield the error C<You cannot create an instance of this type (don't-instantiate)>.
+
+
 =end pod
 
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