The `Sum`

function returns the sum of the argument variables which have
nonmissing values. Examples:

```
total=Sum(x, y, z);
total=Sum(of x1-x10);
total=Sum(of x y z);
```

The first and last example assign `total`

the sum of the variables
`x`

, `y`

, and `z`

which have nonmissing values. Note the
differences in syntax. The last example returns the sum of the variables among ```
x1, ..., x10
```

which have nonmissing values.