It seems the docstring of Transformer class is wrong. The transforms is a 4-tuple, not 3-tuple.
class Transformer(TransformSpec):
"""
Stores transforms (`Transform` classes) and applies them to document
trees. Also keeps track of components by component type name.
"""
def __init__(self, document):
self.transforms = []
"""List of transforms to apply. Each item is a 3-tuple:
``(priority string, transform class, pending node or None)``."""
...
I just made a simple patch for this.
Fixed in r8238. Thanks for reporting and patch.