The translator should automatically
  vectorize (or "matricize") this function.  For example, Sigma([1,2;3,4]) returns {3,7}.
  In contrast, sum([1,2;3,4]) returns 10.
function Sigma
  extends Modelica.Icons.Function;
  input Real u[2] "Vector of size two" annotation(
    Dialog(__Dymola_label = "<html><i>u</i></html>"));
  output Real Sigma "Sum of the first and second entries" annotation(
    Dialog(__Dymola_label = "<html>Σ</html>"));
end Sigma;