/// <inheritdoc/>
/// <exception cref="DimensionMismatchException"> if number of free parameters
/// or orders do not match</exception>
public DerivativeStructure add(DerivativeStructure a)
{
compiler.checkCompatibility(a.compiler);
DerivativeStructure ds = new DerivativeStructure(this);
compiler.add(data, 0, a.data, 0, ds.data, 0);
return(ds);
}