The minus sign ‘−’ is the same width as the plus sign ‘+’, so the button’s transition between the two symbols will look slightly smoother.
If you don’t want to use literal Unicode characters, I can change ‘−’ to `\u2212`. I’m not starting with that suggestion because ‘−’ is easier to read and understand, and if I used `\u2212`, it would probably be necessary to also comment the usage on each line to explain what character is being used.
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