The type OclSummable defines the sum and zero operations used by the Collection::sum iteration. Only types that provide derived sum and zero implementations may be summed.
sum(that : OclSelf) : OclSelf
Return the sum of self and that.
The sum operation should be associative.
zero() : OclSelf
Return the ‘zero’ value of self to initialize a summation.
zero().sum(self) = self.