Cannot negate variable or do simplest arithmetic inside condition statement (in `if`)

[AgainPsychoX]

This does not work:

if $mx < -{?motion_threshold} {

nor

if ?motion_threshold > -$mx {

Workaround:

$mx *= -1
if ?motion_threshold > $mx {
    $mx *= -1
    // Do stuff
}
$mx *= -1

(but it's far away from perfect, since else-if chaining is impossible...)