Add a Cofree.size function.

This should avoid the protocol indirection around `TermType`.

prototype/Doubt/Cofree.swift

@@ -201,4 +201,25 @@ extension Cofree {
 }
 
 
+// MARK: - Size
+
+extension Cofree {
+	/// The count of nodes in the receiver.
+	///
+	/// This is used to compute the cost of patches, such that a patch inserting a very large tree will be charged approximately the same as a very large tree consisting of many small patches.
+	public static func size(term: Cofree) -> Int {
+		switch term.unwrap {
+		case .Leaf:
+			return 1
+		case let .Indexed(a):
+			return a.reduce(0) { $0 + size($1) }
+		case let .Fixed(a):
+			return a.reduce(0) { $0 + size($1) }
+		case let .Keyed(a):
+			return a.reduce(0) { $0 + size($1.1) }
+		}
+	}
+}
+
+
 import Prelude