Gtc

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* Fixed a bug in :func:`core.get_covariance`. The function incorrectly returned zero when the arguments were a single independent elementary uncertain real number.

* Fixed a bug in :func:`core.get_correlation`. The function incorrectly returned zero when the arguments were a single independent elementary uncertain real number.

* :func:`core.result` now raises a warning if applied to a pure number and there is a label specified.

* The content of archive files generated by :func:`persistence.dump`, :func:`persistence.dumps`, :func:`persistence.dump_json` and :func:`persistence.dumps_json` has changed.

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* :func:`reporting.budget` now expects explicit keyword arguments for all options, instead of positional arguments (the names of the previous positional arguments are now the keywords).
* :func:`reporting.budget` takes a new key word ``intermediate``
* :func:`function.implicit` has been added. This finds the uncertain-number solution ``x`` to a user-defined function :math:`fn(x,...) = 0`.

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* Fixed an issue with merging uncertain numbers. The function :func:`type_a.merge` now has a tolerance parameter, which is used to determine whether the arguments ``a`` and ``b`` have equivalent values.

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* Fixed an issue with restoration of archived uncertain numbers. A `RuntimeError` arose if two uncertain numbers, originally created in the same context, were restored to different archive objects in a new common context.

* An attempt to create a file or string representation of an empty archive raises a `RuntimeError`

* Docstrings for :meth:`~.Archive.add` and :meth:`~.Archive.extract` now mention the option of using the name as a look-up key (like a mapping)

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* Fixed an issue with the `r` attribute of uncertain complex numbers, which returns the correlation coefficient between real and imaginary components: the calculation was incorrect (however, :func:`core.get_correlation` gave the correct result).

* Fixed an issue with the calculation of the variance-covariance matrix for an uncertain complex number with finite degrees of freedom: the matrix element for the variance of the real component was sometimes incorrectly returned for the variance of the imaginary component as well.

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* Added support to :mod:`persistence` for archive storage in a JSON format. The new functions are: :func:`persistence.dump_json`, :func:`persistence.dumps_json`, :func:`persistence.load_json` and :func:`persistence.loads_json`