graph_tools.sequences: Add CyclicLadder

graph_tools/sequences.py

 def _init_():
   global \
     FlowerSnark, FlowerSnarkAlt, \
-    Necklace, Flower, \
+    Necklace, Flower, CyclicLadder, \
     GeneralizedPetersen, Petersen
 
   from .base import \
@@ -162,6 +162,27 @@ def _init_():
       return super().graph(n)
 
 
+  @ParametrizedGraphSequence
+  class CyclicLadder:
+    sequence_start = 1
+    D = Gadget.join(
+      [ CUBIC_VERTEX ]*2,
+      [ ((1,3), (2,3)) ],
+      [ (1,1), (2,1), (2,2), (1,2) ]
+    )
+    base_gadget = D
+    step_gadget = D
+
+    step_join = [ ((1,3), (2,2)), ((1,4), (2,1)) ]
+    step_out = [ (1,1), (1,2), (2,3), (2,4) ]
+
+    def __init__(self, crossed=False):
+      if crossed:
+        self.final_join = [ ((1,1), (1,3)), ((1,2), (1,4)) ]
+      else:
+        self.final_join = [ ((1,1), (1,4)), ((1,2), (1,3)) ]
+
+
   Petersen = Gadget.join(
     [ CUBIC_VERTEX ] * 10,
     [