core: Description.delete with another, possibly not matching subject description

lib/rdf/description.ex

@@ -73,6 +73,11 @@ defmodule RDF.Description do
 
   @doc """
   Adds statements to a `RDF.Description`.
+
+  Note: When the statements to be added are given as another `RDF.Description`,
+  the subject must not match subject of the description to which the statements
+  are added. As opposed to that `RDF.Data.merge/2` will produce a `RDF.Graph`
+  containing both descriptions.
   """
   def add(description, statements)
 
@@ -221,6 +226,11 @@ defmodule RDF.Description do
 
   @doc """
   Deletes statements from a `RDF.Description`.
+
+  Note: When the statements to be deleted are given as another `RDF.Description`,
+  the subject must not match subject of the description from which the statements
+  are deleted. If you want to delete only a matching description subject, you can
+  use `RDF.Data.delete/2`.
   """
   def delete(description, statements)
 
@@ -239,6 +249,12 @@ defmodule RDF.Description do
     end
   end
 
+  def delete(description = %RDF.Description{}, other_description = %RDF.Description{}) do
+    Enum.reduce other_description, description, fn ({_, predicate, object}, description) ->
+      delete(description, predicate, object)
+    end
+  end
+
   def delete(description = %RDF.Description{}, predications = %{}) do
     Enum.reduce predications, description, fn ({predicate, objects}, description) ->
       delete(description, predicate, objects)

test/unit/description_test.exs

@@ -270,13 +270,23 @@ defmodule RDF.DescriptionTest do
 
     test "multiple statements with a map of predications",
           %{empty_description: empty_description, description3: description3} do
-      assert Description.delete(empty_description, [{EX.p, [EX.O1, EX.O2]}]) == empty_description
+      assert Description.delete(empty_description, %{EX.p => EX.O1}) == empty_description
       assert Description.delete(description3, %{
                 EX.p1 => EX.O1,
                 EX.p2 => [EX.O2, EX.O3],
                 EX.p3 => [~B<foo>, ~L"bar"],
               }) == Description.new(EX.S, EX.p1, EX.O2)
     end
+
+    test "multiple statements with another description",
+          %{empty_description: empty_description, description1: description1, description3: description3} do
+      assert Description.delete(empty_description, description1) == empty_description
+      assert Description.delete(description3, Description.new(EX.S, %{
+                EX.p1 => EX.O1,
+                EX.p2 => [EX.O2, EX.O3],
+                EX.p3 => [~B<foo>, ~L"bar"],
+              })) == Description.new(EX.S, EX.p1, EX.O2)
+    end
   end