core: put statements from one graph into another

lib/rdf/graph.ex

@@ -187,11 +187,18 @@ defmodule RDF.Graph do
 
   def put(%RDF.Graph{name: name, descriptions: descriptions},
           %Description{subject: subject} = description) do
-    description = if existing_description = descriptions[subject],
-      do:   Description.put(existing_description, description),
-      else: description
     %RDF.Graph{name: name,
-               descriptions: Map.put(descriptions, subject, description)}
+      descriptions:
+        Map.update(descriptions, subject, description, fn current ->
+          current |> Description.put(description)
+        end)
+    }
+  end
+
+  def put(graph, %RDF.Graph{descriptions: descriptions}) do
+    Enum.reduce descriptions, graph, fn ({_, description}, graph) ->
+      put(graph, description)
+    end
   end
 
   def put(%RDF.Graph{} = graph, statements) when is_map(statements) do

test/unit/graph_test.exs

@@ -234,6 +234,26 @@ defmodule RDF.GraphTest do
         assert graph_includes_statement?(g, {EX.S2, EX.P2, EX.O3})
         assert graph_includes_statement?(g, {EX.S2, EX.P2, EX.O4})
     end
+
+    test "a Graph" do
+      g =
+        Graph.new([
+          {EX.S1, EX.P1, EX.O1},
+          {EX.S1, EX.P3, EX.O3},
+          {EX.S2, EX.P2, EX.O2},
+        ])
+        |> RDF.Graph.put(Graph.new([
+          {EX.S1, EX.P3, EX.O4},
+          {EX.S2, EX.P2, bnode(:foo)},
+          {EX.S3, EX.P3, EX.O3}
+        ]))
+
+      assert Graph.triple_count(g) == 4
+      assert graph_includes_statement?(g, {EX.S1, EX.P1, EX.O1})
+      assert graph_includes_statement?(g, {EX.S1, EX.P3, EX.O4})
+      assert graph_includes_statement?(g, {EX.S2, EX.P2, bnode(:foo)})
+      assert graph_includes_statement?(g, {EX.S3, EX.P3, EX.O3})
+    end
   end
 
   test "pop a triple" do