diff --git a/Jellyfin.Plugin.SmartPlaylist/Lisp/Interpreter.cs b/Jellyfin.Plugin.SmartPlaylist/Lisp/Interpreter.cs
index 8dd89a9..e2a4e74 100644
--- a/Jellyfin.Plugin.SmartPlaylist/Lisp/Interpreter.cs
+++ b/Jellyfin.Plugin.SmartPlaylist/Lisp/Interpreter.cs
@@ -1,4 +1,3 @@
-using System;
 using System.Reflection;
 
 using Jellyfin.Plugin.SmartPlaylist.Lisp.Compiler;
@@ -7,7 +6,99 @@ namespace Jellyfin.Plugin.SmartPlaylist.Lisp {
     using Function = Func<IList<Expression>, Expression>;
     using FunctionLater = Func<Executor, IList<Expression>, Expression>;
 
-    public class Environment : Dictionary<string, Expression> {}
+    public class Procedure : Expression {
+        private Compiler.List _parameters;
+        private Expression _body;
+        public Procedure(Compiler.List parameters, Expression body) {
+            _parameters = parameters;
+            _body = body;
+        }
+        private static IEnumerable<(T1, T2)> Zip<T1, T2>(IEnumerable<T1> a, IEnumerable<T2> b) {
+            using (var e1 = a.GetEnumerator()) using (var e2 = b.GetEnumerator()) {
+                while (e1.MoveNext() && e2.MoveNext()) {
+                    yield return (e1.Current, e2.Current);
+                }
+            }
+        }
+        public override int GetHashCode() {
+            int hash = 17;
+            hash *= 23;
+            hash += _parameters.GetHashCode();
+            hash *= 23;
+            hash += _body.GetHashCode();
+            return hash;
+        }
+        public override bool Equals(Expression? other) {
+            if (other is Procedure other_p) {
+                return _parameters == other_p._parameters && _body == other_p._body;
+            }
+            return false;
+        }
+        public override object Inner() {
+            throw new ApplicationException("This is not sensible");
+        }
+        public override string ToString() {
+            return $"(lambda {_parameters} {_body})";
+        }
+
+        public Expression Call(Executor e, IList<Expression> args) {
+            var p = _parameters.expressions.Select(x => x.Inner().ToString()).ToList();
+            Executor new_e = new Executor(new SubEnvironment(e.environment), e.builtins, e.builtinsLater);
+            foreach (var tuple in Zip<string, Expression>(p, args)) {
+                new_e.environment.Set(tuple.Item1, tuple.Item2);
+            }
+            return new_e.eval(_body);
+        }
+    }
+
+    public interface IEnvironment<K, V> {
+        public V Get(K k);
+        public void Set(K k, V v);
+        public IEnvironment<K, V>? Find(K k);
+    }
+
+    public class Environment : Dictionary<string, Expression>, IEnvironment<string, Expression> {
+        public Expression? Get(string k) {
+            if (TryGetValue(k, out Expression v)) {
+                return v;
+            }
+            return null;
+        }
+        public void Set(string k, Expression v) {
+            Add(k, v);
+        }
+
+        public IEnvironment<string, Expression>? Find(string k) {
+            if (ContainsKey(k)) {
+                return this;
+            }
+            return null;
+        }
+    }
+
+    public class SubEnvironment : Dictionary<string, Expression>, IEnvironment<string, Expression> {
+        private IEnvironment<string, Expression> _super;
+        public SubEnvironment(IEnvironment<string, Expression> super) {
+            _super = super;
+        }
+        public Expression? Get(string k) {
+            if (TryGetValue(k, out Expression v)) {
+                return v;
+            }
+            return null;
+        }
+        public void Set(string k, Expression v) {
+            Add(k, v);
+        }
+
+        public IEnvironment<string, Expression>? Find(string k) {
+            if (ContainsKey(k)) {
+                return this;
+            }
+            return _super.Find(k);
+        }
+    }
+
     public class Builtins : Dictionary<string, Function> {
         public Builtins() : base() {
             this["+"] = _add;
@@ -237,6 +328,7 @@ namespace Jellyfin.Plugin.SmartPlaylist.Lisp {
         public BuiltinsLater() : base() {
             this["if"] = _if;
             this["define"] = _define;
+            this["lambda"] = _lambda;
             this["apply"] = _apply;
             this["and"] = _and;
             this["or"] = _or;
@@ -246,9 +338,13 @@ namespace Jellyfin.Plugin.SmartPlaylist.Lisp {
             return e.eval(args[1 + (test ? 0 : 1)]);
         }
         private static Expression _define(Executor e, IList<Expression> args) {
-            e.environment[((Symbol) args[0]).name] = e.eval(args[1]);
+            var refname = ((Symbol) args[0]).name;
+            e.environment.Set(refname, e.eval(args[1]));
             return new Compiler.Boolean(false); // NOOP
         }
+        private static Expression _lambda(Executor e, IList<Expression> args) {
+            return new Procedure((Compiler.List) args[0], args[1]);
+        }
         private static Expression _apply(Executor e, IList<Expression> args) {
             if (args[0].GetType() != typeof(Symbol)) {
                 throw new ApplicationException();
@@ -279,10 +375,10 @@ namespace Jellyfin.Plugin.SmartPlaylist.Lisp {
     }
 
     public class Executor {
-        Environment _environment;
+        IEnvironment<string, Expression> _environment;
         Builtins _builtins;
         BuiltinsLater _builtinsLater;
-        public Executor(Environment environment, Builtins builtins, BuiltinsLater builtinsLater) {
+        public Executor(IEnvironment<string, Expression> environment, Builtins builtins, BuiltinsLater builtinsLater) {
             _environment = environment;
             _builtins = builtins;
             _builtinsLater = builtinsLater;
@@ -293,13 +389,13 @@ namespace Jellyfin.Plugin.SmartPlaylist.Lisp {
             _builtinsLater = new BuiltinsLater();
         }
 
-        public Environment environment { get => _environment; }
+        public IEnvironment<string, Expression> environment { get => _environment; }
         public Builtins builtins { get => _builtins; }
         public BuiltinsLater builtinsLater { get => _builtinsLater; }
 
         public Expression EvalFunction(Symbol fcname, IList<Expression> args) {
-            if (_environment.ContainsKey(fcname.name)) {
-                Expression first = environment[fcname.name];
+            if (_environment.Find(fcname.name) is IEnvironment<string, Expression> _e) {
+                Expression? first = _e.Get(fcname.name);
                 return new List(new []{first}.ToList()) + new List(args.Select(x => eval(x)).ToList());
             }
             if (_builtins.ContainsKey(fcname.name)) {
@@ -317,21 +413,31 @@ namespace Jellyfin.Plugin.SmartPlaylist.Lisp {
         public Expression eval(Expression expression) {
             switch (expression) {
                 case Symbol s:
-                    return _environment[s.name];
+                    return _environment.Find(s.name).Get(s.name);
                 case Compiler.Boolean b:
                     return b;
                 case Integer i:
                     return i;
                 case Compiler.String s:
                     return s;
+                case Procedure p:
+                    return p;
                 case List list:
                     // do we really want to allow shadowing of builtins?
                     if (list.expressions[0].GetType() == typeof(Symbol)) {
-                        return EvalFunction((Symbol) list.expressions[0], list.expressions.Skip(1).ToList());
+                        return eval(EvalFunction((Symbol) list.expressions[0], list.expressions.Skip(1).ToList()));
                     }
-                    return new List(list.expressions.Select(x => eval(x)).ToList());
+                    if (list.expressions[0].GetType() == typeof(Procedure)) {
+                        Procedure procedure = (Procedure) list.expressions[0];
+                        return eval(procedure.Call(this, list.expressions.Skip(1).ToList()));
+                    }
+                    var l = new List(list.expressions.Select(x => eval(x)).ToList());
+                    if (l.expressions[0].GetType() == typeof(Procedure)) {
+                        return eval(l);
+                    }
+                    return l;
             }
-            throw new ApplicationException("Not handled case");
+            throw new ApplicationException($"Not handled case '{expression}'");
         }
         public Expression eval(Parser p) {
             return eval(p.parse());
diff --git a/Jellyfin.Plugin.SmartPlaylist/ScheduledTasks/GeneratePlaylist.cs b/Jellyfin.Plugin.SmartPlaylist/ScheduledTasks/GeneratePlaylist.cs
index 34925d6..8405967 100644
--- a/Jellyfin.Plugin.SmartPlaylist/ScheduledTasks/GeneratePlaylist.cs
+++ b/Jellyfin.Plugin.SmartPlaylist/ScheduledTasks/GeneratePlaylist.cs
@@ -105,9 +105,9 @@ namespace Jellyfin.Plugin.SmartPlaylist.ScheduledTasks {
             List<Guid> results = new List<Guid>();
             Expression expression = new Parser(StringTokenStream.generate(smartPlaylist.Program)).parse();
             Executor executor = new Executor();
-            executor.environment["user"] = new Lisp_Object(user);
+            executor.environment.Set("user", new Lisp_Object(user));
             foreach (var i in items) {
-                executor.environment["item"] = new Lisp_Object(i);
+                executor.environment.Set("item", new Lisp_Object(i));
                 var r = executor.eval(expression);
                 _logger.LogTrace("Item {0} evaluated to {1}", i, r.ToString());
                 if (r is Lisp_Boolean r_bool) {
diff --git a/Jellyfin.Plugin.SmartPlaylist/jellyfin-smart-playlist.csproj b/Jellyfin.Plugin.SmartPlaylist/jellyfin-smart-playlist.csproj
index ae98220..604ef49 100644
--- a/Jellyfin.Plugin.SmartPlaylist/jellyfin-smart-playlist.csproj
+++ b/Jellyfin.Plugin.SmartPlaylist/jellyfin-smart-playlist.csproj
@@ -10,6 +10,7 @@
   <ItemGroup>
     <PackageReference Include="Jellyfin.Controller" Version="10.9.7" />
     <PackageReference Include="Jellyfin.Model" Version="10.9.7" />
+    <PackageReference Include="OneOf" Version="3.0.271" />
   </ItemGroup>
 
 </Project>
diff --git a/Tests/Tests.cs b/Tests/Tests.cs
index e102eb3..622f376 100644
--- a/Tests/Tests.cs
+++ b/Tests/Tests.cs
@@ -113,11 +113,27 @@ namespace Tests
         public static void ObjectTest() {
             Executor e = new Executor();
             Expression r;
-            e.environment["o"] = new Lisp_Object(new O(5, false));
+            e.environment.Set("o", new Lisp_Object(new O(5, false)));
             r = e.eval("(haskeys o 'i' 'b')");
             Assert.Equal(((Lisp_Boolean)r).value, true);
             r = e.eval("(getitems o 'i' 'b')");
             Assert.Equal(string.Format("{0}", r), "(5 nil)");
         }
+
+        [Fact]
+        public static void ProcedureTest() {
+            Executor e = new Executor();
+            Expression r;
+            r = e.eval("((lambda (a) (* a a)) 2)");
+            Assert.Equal(string.Format("{0}", r), "4");
+
+            r = e.eval("(begin (define mull (lambda (a) (* a a))) (mull 3))");
+            Assert.Equal(string.Format("{0}", r), "9");
+
+            //r = e.eval("""
+            //(begin (define pi 3.1415) 1)
+            //""");
+            //Assert.Equal(string.Format("{0}", r), "1");
+        }
     }
 }