Nemeth: Missing multipurpose indicator for +-

[NSoiffer]

Rule 134 (p132) in the green book is about plus followed by minus and vice-versa. In those cases, a multipurpose indicator is needed to prevent the interpretation being ± (etc).

Here are are a few samples from the green book and NFB provisional guidelines:

<math><mo>+</mo><mn>2</mn><mo>-</mo><mo>+</mo><mn>3</mn></math>
⠬⠆⠤⠐⠬⠒

<math><mn>10</mn><mo>+</mo><mo>-</mo><mn>5</mn></math>
⠼⠂⠴⠬⠐⠤⠢

<math><mn>10</mn><mo>-</mo><mo>-</mo><mn>5</mn></math>
⠼⠂⠴⠤⠐⠤⠢

Note the presence of the ⠐ between the "+" and "-".