fix get_can_dot for output

cotengra/core.py

@@ -217,7 +217,9 @@ def __init__(
         for ix in self.output:
             self.appearances[ix] = self.appearances.get(ix, 0) + 1
 
-        #
+        # this stores potentialy preprocessing steps that are not part of the
+        # main contraction tree, but assumed to have been applied, for example
+        # tracing or summing over indices that appear only once
         self.preprocessing = {}
 
         # mapping of parents to children - the core binary tree object
@@ -738,6 +740,13 @@ def has_preprocessing(self):
             self.get_legs(node)
         return bool(self.preprocessing)
 
+    def has_hyper_indices(self):
+        """Check if there are any 'hyper' indices in the contraction, i.e.
+        indices that don't appear exactly twice, when considering the inputs
+        and output.
+        """
+        return any(ix_count != 2 for ix_count in self.appearances.values())
+
     @cached_node_property("legs")
     def get_legs(self, node):
         """Get the effective 'outer' indices for the collection of tensors
@@ -795,15 +804,7 @@ def get_can_dot(self, node):
         """
         l, r = self.children[node]
         sp, sl, sr = map(self.get_legs, (node, l, r))
-
-        srl_symmdiff = sl.copy()
-        for ix, ix_count in sr.items():
-            if ix in srl_symmdiff:
-                srl_symmdiff.pop(ix)
-            else:
-                srl_symmdiff[ix] = ix_count
-
-        return srl_symmdiff == sp
+        return set(sp) == set(sl).symmetric_difference(sr)
 
     @cached_node_property("inds")
     def get_inds(self, node):