Structured-data

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Changed
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- Actually, the facade was working, I was just confused. Restored the facade.

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Changed
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- Removed the facade.
- Added stability guarantee to Ctor.

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Changed
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- Rewrote the facade.

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Changed
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- Tried to put up a facade for type analysis. It didn't work.

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Added
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- ``Bind`` class for attaching extra data to a match structure.
- PEP 561 support.

Changed
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- As-patterns are now formed with indexing instead of the ` operator.
- ``AttrPattern`` and ``DictPattern`` now take keyword arguments instead of a ``dict`` argument, and form new versions of themselves with an ``alter`` method.
- Actually. Change ``DictPattern`` back, stop trying to keep these things in synch.

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Added
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- ``AttrPattern`` and ``DictPattern`` classes that take a ``dict`` argument and perform destructuring match against arbitrary objects, and mappings, respectively.

Changed
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- Added special handling for matching AsPatterns against different AsPatterns. This is subject to change, as it's definitely an edge case.