Update README

README.md

@@ -79,16 +79,42 @@ extension P {
 }
 
 struct PBuffer {
-    var startIndex: UnsafeHeterogeneousBuffer.Index { contents.startIndex }
-    var endIndex: UnsafeHeterogeneousBuffer.Index { contents.endIndex }
-
     var contents: UnsafeHeterogeneousBuffer
+
+    mutating func append<T>(_ t: T) -> UnsafeHeterogeneousBuffer.Index where T: P {
+        contents.append(t, vtable: _VTable<T>.self)
+    }
+
+    subscript<T>(_ type: T.Type) -> UnsafeMutablePointer<T>? where T: P {
+        guard !contents.isEmpty else { return nil }
+        for elelement in contents  {
+            guard elelement.hasType(type) else {
+                continue
+            }
+            return elelement.body(as: type)
+        }
+        return nil
+    }
 
     typealias Index = UnsafeHeterogeneousBuffer.Index
 
     struct Element {
         var base: UnsafeHeterogeneousBuffer.Element
     }
+
+    var startIndex: UnsafeHeterogeneousBuffer.Index { contents.startIndex }
+
+    var endIndex: UnsafeHeterogeneousBuffer.Index { contents.endIndex }
+
+    var isEmpty: Bool { contents.isEmpty }
+
+    subscript(position: UnsafeHeterogeneousBuffer.Index) -> Element {
+        _read { yield Element(base: contents[position]) }
+    }
+
+    func index(after i: UnsafeHeterogeneousBuffer.Index) -> UnsafeHeterogeneousBuffer.Index {
+        contents.index(after: i)
+    }
 
     private class VTable: _UnsafeHeterogeneousBuffer_VTable {
         class func modify(elt: UnsafeHeterogeneousBuffer.Element, inputs: inout Int) {}