[Suggestion] The definition name dropdown should default to the first undefined matrix

[DoctorDalek1963]

Summary

When defining a matrix, the dropdown should default to the first matrix which has not been previously defined. If all matrices already have a definition, then use A.

Example

It is a common problem that a user will accidentally overwrite A instead of defining a new matrix, because they didn't remember to choose a new name.

This would also solve the issue of having to remember which names you'd already used, since the default name would be undefined.

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