If you put a value in the "symbol" field of an instruction with an
address operand, you're establishing a symbolic reference to a label
that may be at a different address than the numeric value of the
operand. But if you then hit the "edit label" button, you'll edit
the label at the *numeric* reference address, which can be confusing.
We now disable the create/edit label button when the format has been
set to "symbol".
The button is just a short-cut, so disabling it doesn't prevent the
user from doing anything.
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