class A { public Prop1 : int { get{...} }
class B: A { public Prop2 : A { get{...} }

match (x)
{
  | B{ Prop1=1; Prop2=B { Prop1=2; }; } => ...
  | A{ Prop1=1; } => ...
}

VD>class A { public Prop1 : int { get{...} }
VD>class B: A { public Prop2 : A { get{...} }

VD>match (x)
VD>{
VD>  | B{ Prop1=1; Prop2=B { Prop1=2; }; } => ...
VD>  | A{ Prop1=1; } => ...
VD>}
VD>

type Forest a = [Tree a] 

data Tree a = Node {
                    rootLabel :: a 
                    subForest :: (Forest a) 
                   } 

drawTree :: Tree String -> String
drawTree  = unlines . draw

draw :: Tree String -> [String]
draw (Node x ts0) = x : drawSubTrees ts0
  where -- разбираем по конструкторам списка
        drawSubTrees []     = []
        drawSubTrees [t]    = "|" : shift "`- " "   " (draw t)
        drawSubTrees (t:ts) = "|" : shift "+- " "|  " (draw t) ++ drawSubTrees ts

        shift first other = zipWith (++) (first : repeat other)

drawTree (Node "r" [(Node "a" []), (Node "b" [])])

r
|
+- a
|
`- b

/// Описывает ветку дерева «красно-черного» дерева.
public variant Node[T] : IEnumerable[T] where T : IComparable[T] 
{
  | Red   { key : T; lchild : Node[T]; rchild : Node[T]; }
  | Black { key : T; lchild : Node[T]; rchild : Node[T]; }
  | Leaf

  /// Подсчет количества элементов.
  public Count : int 
  {
    get
    {
      match (this)
      {
        | Red(_, left, right) | Black(_, left, right) =>
          1 + left.Count + right.Count
        | _ => 0
      }
    }
  }

VD>>>class A { public Prop1 : int { get{...} }
VD>>>class B: A { public Prop2 : A { get{...} }

VD>>>match (x)
VD>>>{
VD>>>  | B{ Prop1=1; Prop2=B { Prop1=2; }; } => ...
VD>>>  | A{ Prop1=1; } => ...
VD>>>}
VD>>>

| B { Prop1=1; Prop2=B { Prop1=2; } as b; } => b.Prop2 *= 2;

| B { Prop1=1; Prop2=B { Prop1=x; }; } => x

VD>| B { Prop1=1; Prop2=B { Prop1=2; } as b; } => b.Prop2 *= 2;
VD>

VD>| B { Prop1=1; Prop2=B { Prop1=x; }; } => x
VD>

if (typeof(x)=B && x.Prop1=1 && typeof(x.Prop2)=B && x.Prop2.Prop1=2)
    {x.(добрались через "." до нужного свойства)*=2;}
else if (typeof(x)=A && x.Prop1=1)
    {...}

D>if (typeof(x)=B && x.Prop1=1 && typeof(x.Prop2)=B && x.Prop2.Prop1=2)
D>    {x.(добрались через "." до нужного свойства)*=2;}
D>else if (typeof(x)=A && x.Prop1=1)
D>    {...}
D>

D>Чуть длиннее, но ИМХО требования к x выражены более ясно

//  | B{ Prop1=1; Prop2=B { Prop1=2; }; } => ...
B b1 = x as B.
if (b1 != null && b1.Prop1 == 1)
{
  B b2 = b1.Prop2 as B;
    if (b2 != null && b2.Prop1 == 2)
    {
      ...
    }
}