public static GenTensor <T, TWrapper> PiecewiseSubtract(
     T a, GenTensor <T, TWrapper> b, Threading threading)
 => Constructors <T, TWrapper> .CreateTensor(b.Shape, ind =>
                                             default(TWrapper).Subtract(a, b[ind]), threading);
 public static GenTensor <T, TWrapper> PiecewiseSubtract(GenTensor <T, TWrapper> a,
                                                         T b, Threading threading)
 => Constructors <T, TWrapper> .CreateTensor(a.Shape, ind =>
                                             default(TWrapper).Subtract(a[ind], b), threading);