core: normalize internal blank node representation

lib/rdf/blank_node.ex

@@ -1,13 +1,22 @@
 defmodule RDF.BlankNode do
   @moduledoc """
+  An RDF blank node, also known as an anonymous or unlabeled node.
+
+  see <https://www.w3.org/TR/rdf11-primer/#section-blank-node>
+  and <https://www.w3.org/TR/rdf11-concepts/#section-blank-nodes>
   """
 
   defstruct [:id]
 
   @type t :: module
 
-  def new, do: %RDF.BlankNode{id: make_ref()}
-  def new(id) when is_atom(id) or is_binary(id) or is_integer(id),
+  def new,
+    do: new(make_ref())
+  def new(id) when is_binary(id),
     do: %RDF.BlankNode{id: id}
+  def new(id) when is_reference(id),
+    do: id |> :erlang.ref_to_list |> to_string |> String.replace(~r/\<|\>/, "") |> new
+  def new(id) when is_atom(id) or is_integer(id),
+    do: id |> to_string |> new
 
 end