A plus and a minus don't cancel each other out though. Once a minus is introduced all of the plus signs can be ignored (ie. $calc(2 - 3) could also be expressed as $calc(2 + -3) or any number of pluses without changing the result)
$calc(1 +++--++-- 2) is equivalent to $calc(1 ---- 2) which might be easier to think of as this: $calc(1 - (-(-(-2)))). Another way to think about it is that two minuses equal a plus, so $calc(1 ---- 2) is the same as $calc(1 ++ 2), or more simply: $calc(1 + 2).
