@private Together with the example group metadata hash default block, provides backwards compatibility for the old `:example_group` key. In RSpec 2.x, the computed keys of a group’s metadata were exposed from a nested subhash keyed by `[:example_group]`, and then the parent group’s metadata was exposed by sub-subhash keyed by `[:example_group]`.
In RSpec 3, we reorganized this to that the computed keys are exposed directly of the group metadata hash (no nesting), and `:parent_example_group` returns the parent group’s metadata.
Maintaining backwards compatibility was difficult: we wanted `:example_group` to return an object that:
* Exposes the top-level metadata keys that used to be nested under `:example_group`. * Supports mutation (rspec-rails, for example, assigns `metadata[:example_group][:described_class]` when you use anonymous controller specs) such that changes are written back to the top-level metadata hash. * Exposes the parent group metadata as `[:example_group][:example_group]`.
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