[Python] Rational number generator (Python)

1. 
   Why is it getting rid of the [1,3],[3,2] entry and replacing it with [2,3],[3,1]?
   
   
2. 
   Note, [2,3],[3,1] is the correct branch, but it should not override the previous entry, it should be appended to it.
   
   def rational():
   treecount = 0
   tree = [[[1,1]]]
   left=[0,0]
   right=[0,0]
   while(True):
   treecount+=1
   tree.append([])
   left=[0,0]
   right=[0,0]
   for z in range(len(tree[treecount-1])):
   left[0] = tree[treecount-1][z][0]
   right[0] = tree[treecount-1][z][0] + tree[treecount-1][z][1]
   left[1] = tree[treecount-1][z][1] + tree[treecount-1][z][0]
   right[1] = tree[treecount-1][z][1]
   tree[treecount].append(left)
   tree[treecount].append(right)
   yield( tree)

Output looks like this:

[[[1, 1]], [[1, 2], [2, 1]]] [[[1, 1]], [[1, 2], [2, 1]], [[1, 3], [3, 2]]] [[[1, 1]], [[1, 2], [2, 1]], [[2, 3], [3, 1], [2, 3], [3, 1]]]

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