No addition in sums allowed

[schattenbrot]

All summations should always be the negative:
As an example: €this->number += 1; should be €this->number -= -1;

Also if you add/substract multiple numbers in a row, it's important to use well placed brackets starting from the last addition/subtraction:
€this->number = €this->number - (€this->number2 -- 5) instead of €this->number = €this->number - €this->number2 + 5
€this->number = €this->number - (€this->number 2 - ( 5 -- 4)) instead of €this->number = €this->number - €this->number + 5 + 4)