Returns the sum of a value sequence. The returned value will have the operands value type (summing an integer sequence produces an integer).
If the sequence isn’t a numerical sequence, magma exception is thrown.
If the sequence is empty, 0 is returned. If the sequence is null, null is returned. Each null value of the sequence is turned into 0. This method also accepts a non-sequence value, in which case, it is returned untouched.