Use parens again if negative is factored out. (#455)

[dpvc]

This PR address issue #455, which causes the display for some reduced formulas be incorrect (parens are missing). The expression evaluates properly, but doesn't print properly. This is due to a setting that was trying to prevent (-2)x from showing as -(2*x) when it is reduced.

The reduction works by pulling negatives out and cancelling when possible, so -5/(-2*x) becomes -(5/(2*x)), then -(5/(-(2*x))), then -(-(5/(2x))and finally5/(2x). But at the (-2x)to-(2x)reduction, the parens are suppressed (so it would print as-2x, and the further reduction didn't change that, so you got5/2xinstead of5/(2*x)`. This PR removes the no-paren marker when the negation is moved further out, allowing the parens to be included again.

[Alex-Jordan]

I tested with the original problem and a few more expressions I thought might be affected. All I can see is that it fixes the original issue without any side effects. So I recommend for merging.

[drgrice1]

This checks out for me too.