Added BTree.PreviousOrCurrent* method family

BTree.cs

@@ -259,6 +259,44 @@ namespace ln.collections
             return false;
         }
 
+        public K PreviousOrCurrent(K current)
+        {
+            if (!TryGetPreviousOrCurrent(current, out K previousOrCurrent))
+                throw new KeyNotFoundException();
+            return previousOrCurrent;
+        }
+        public bool TryGetPreviousOrCurrent(K current, out K previousOrCurrent)
+        {
+            if (Empty)
+            {
+                previousOrCurrent = default(K);
+                return false;
+            }
+
+            TreeNode next = FindFirstGE(current);
+            if (next == null)
+                previousOrCurrent = Last();
+            else
+                previousOrCurrent = Previous(next).Key;
+
+            return true;
+        }
+        public bool TryGetPreviousOrCurrentValue(K current, out V previousOrCurrentValue)
+        {
+            if (Empty)
+            {
+                previousOrCurrentValue = default(V);
+                return false;
+            }
+
+            TreeNode next = FindFirstGE(current);
+            if (next == null)
+                previousOrCurrentValue = LastValue();
+            else
+                previousOrCurrentValue = Previous(next).Value;
+
+            return true;
+        }