[Pharo-users] Number is not accepted by UI

Hi, I am using latest Pharo 3.0 + vm. I have a method width with width ^ 115 * self ratio. in my understanding it returns a Number. When I try to create a method negatedWidth ^ - (self width). the UI tells me that only Numbers may be negated. I am not able to save the method. Am I on a wrong path or should that work and if how can I avoid the UI error message? Regards Max