misc.graph_to_gadget: allow gadgets of non-zero size

graph_tools/misc.py

@@ -42,9 +42,12 @@ def _init_():
     return (b.eval(CircuitDoubleCover), 2**(G.num_verts() // 2) // 2)
 
 
-  def graph_to_gadget(G, decomposition = None):
+  def graph_to_gadget(G, decomposition = None, allow_non_graph = False):
     """Transform graph into a Gadget.
 
+    If allow_non_graph is True, allow transforming a part of the
+    graph into a gadget of non-zero size.
+
     EXAMPLES:
     >>> from sage.all import *
     >>> from .parameters import CircuitDoubleCover, UnderlayingGraph
@@ -106,7 +109,7 @@ def _init_():
       return (Gadget.join([ g for g, _ in gadgets ], joins, outs), gadget_verts)
 
     b, _ = process(decomposition)
-    assert b.is_graph()
+    assert allow_non_graph or b.is_graph()
     return b