add meta:choose/*

Ergo/Interpreter/Libraries/Meta/Meta.cs

@@ -14,6 +14,7 @@ public override IEnumerable<BuiltIn> GetExportedBuiltins() => Enumerable.Empty<B
         .Append(new FindAll())
         .Append(new SetOf())
         .Append(new SetupCallCleanup())
+        .Append(new Choose())
         ;
     public override IEnumerable<InterpreterDirective> GetExportedDirectives() => Enumerable.Empty<InterpreterDirective>()
         ;

Ergo/Runtime/Built-Ins/Meta/Choose.cs

@@ -0,0 +1,20 @@
+namespace Ergo.Runtime.BuiltIns;
+
+public sealed class Choose : BuiltIn
+{
+    public Random Rng { get; set; } = new Random();
+    private readonly Call CallInst = new();
+
+    public Choose()
+        : base("", new("choose"), default, WellKnown.Modules.Meta)
+    {
+    }
+
+    public override ErgoVM.Op Compile() => vm =>
+    {
+        var arg = vm.Args[Rng.Next(vm.Arity)];
+        vm.Arity = 1;
+        vm.SetArg(0, arg);
+        CallInst.Compile()(vm);
+    };
+}