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.

conformsTo
`OclAny`

**Operations**

`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.