/// <summary>
/// Linear combination constructor.
/// The derivative structure built will be a1 * ds1 + a2 * ds2
/// </summary>
/// <param name="a1">first scale factor</param>
/// <param name="ds1">first base (unscaled) derivative structure</param>
/// <param name="a2">second scale factor</param>
/// <param name="ds2">second base (unscaled) derivative structure</param>
/// <exception cref="DimensionMismatchException"> if number of free parameters
/// or orders are inconsistent</exception>
public DerivativeStructure(double a1, DerivativeStructure ds1, double a2, DerivativeStructure ds2)
: this(ds1.compiler)
{
compiler.checkCompatibility(ds2.compiler);
compiler.linearCombination(a1, ds1.data, 0, a2, ds2.data, 0, data, 0);
}