# System

``` diff
 namespace System {
     public sealed class AppDomain : MarshalByRefObject {
+        public PermissionSet PermissionSet { get; }
     }
     public sealed class String : ICloneable, IComparable, IComparable<string>, IConvertible, IEnumerable, IEnumerable<char>, IEquatable<string> {
+        public static String Concat(ReadOnlySpan<char> str0, ReadOnlySpan<char> str1);
+        public static String Concat(ReadOnlySpan<char> str0, ReadOnlySpan<char> str1, ReadOnlySpan<char> str2);
+        public static String Concat(ReadOnlySpan<char> str0, ReadOnlySpan<char> str1, ReadOnlySpan<char> str2, ReadOnlySpan<char> str3);
     }
 }
```

