- `Y = sum (X)`
- `Y = sum (X, DIM)`

Sum of elements along dimension `DIM`.

If `DIM` is omitted, it defaults to the first non-singleton dimension.

See also: `cumsum`, `sumsq`, `prod`.

### References

- https://www.mathworks.com/help/matlab/ref/sum.html
- https://octave.sourceforge.io/octave/function/um.html