Mantis#1314. Thank you kindly, Kinoc for YieldProlog.

I have added everything *except* the patch to 
.../LSL/Compiler.cs. The Compiler.cs patch has a 
namespace issue. Lets make a second patch to close
the gap.

OpenSim/Region/ScriptEngine/DotNetEngine/Compiler/YieldProlog/Variable.cs

@@ -0,0 +1,196 @@
+/*
+ * Copyright (C) 2007-2008, Jeff Thompson
+ * 
+ * All rights reserved.
+ * 
+ * Redistribution and use in source and binary forms, with or without 
+ * modification, are permitted provided that the following conditions are met:
+ * 
+ *     * Redistributions of source code must retain the above copyright 
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright 
+ *       notice, this list of conditions and the following disclaimer in the 
+ *       documentation and/or other materials provided with the distribution.
+ *     * Neither the name of the copyright holder nor the names of its contributors 
+ *       may be used to endorse or promote products derived from this software 
+ *       without specific prior written permission.
+ * 
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+ * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+ * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+using System;
+using System.Collections;
+using System.Collections.Generic;
+
+namespace OpenSim.Region.ScriptEngine.DotNetEngine.Compiler.YieldProlog
+{
+    public interface IUnifiable
+    {
+        IEnumerable<bool> unify(object arg);
+        void addUniqueVariables(List<Variable> variableSet);
+        object makeCopy(Variable.CopyStore copyStore);
+        bool termEqual(object term);
+        bool ground();
+    }
+
+    public class Variable : IUnifiable
+    {
+        // Use _isBound separate from _value so that it can be bound to any value,
+        //   including null.
+        private bool _isBound = false;
+        private object _value;
+
+        public object getValue()
+        {
+            if (!_isBound)
+                return this;
+
+            object result = _value;
+            while (result is Variable)
+            {
+                if (!((Variable)result)._isBound)
+                    return result;
+
+                // Keep following the Variable chain.
+                result = ((Variable)result)._value;
+            }
+
+            return result;
+        }
+
+        public IEnumerable<bool> unify(object arg)
+        {
+            if (!_isBound)
+            {
+                _value = YP.getValue(arg);
+                if (_value == this)
+                    // We are unifying this unbound variable with itself, so leave it unbound.
+                    yield return false;
+                else
+                {
+                    _isBound = true;
+                    try
+                    {
+                        yield return false;
+                    }
+                    finally
+                    {
+                        // Remove the binding.
+                        _isBound = false;
+                    }
+                }
+            }
+            else
+            {
+                foreach (bool l1 in YP.unify(this, arg))
+                    yield return false;
+            }
+        }
+
+        public override string ToString()
+        {
+            object value = getValue();
+            if (value == this)
+                return "Variable";
+            else
+                return getValue().ToString();
+        }
+
+        /// <summary>
+        /// If bound, call YP.addUniqueVariables on the value.  Otherwise, if this unbound
+        /// variable is not already in variableSet, add it.
+        /// </summary>
+        /// <param name="variableSet"></param>
+        public void addUniqueVariables(List<Variable> variableSet)
+        {
+            if (_isBound)
+                YP.addUniqueVariables(getValue(), variableSet);
+            else
+            {
+                if (variableSet.IndexOf(this) < 0)
+                    variableSet.Add(this);
+            }
+        }
+
+        /// <summary>
+        /// If bound, return YP.makeCopy for the value, else return copyStore.getCopy(this).
+        /// However, if copyStore is null, just return this.
+        /// </summary>
+        /// <param name="copyStore"></param>
+        /// <returns></returns>
+        public object makeCopy(Variable.CopyStore copyStore)
+        {
+            if (_isBound)
+                return YP.makeCopy(getValue(), copyStore);
+            else
+                return copyStore == null ? this : copyStore.getCopy(this);
+        }
+
+        public bool termEqual(object term)
+        {
+            if (_isBound)
+                return YP.termEqual(getValue(), term);
+            else
+                return this == YP.getValue(term);
+        }
+
+        public bool ground()
+        {
+            if (_isBound)
+                // This is usually called by YP.ground which already did getValue, so this
+                //   should never be reached, but check anyway.
+                return YP.ground(getValue());
+            else
+                return false;
+        }
+
+        /// <summary>
+        /// A CopyStore is used by makeCopy to track which Variable objects have
+        /// been copied.
+        /// </summary>
+        public class CopyStore
+        {
+            private List<Variable> _inVariableSet = new List<Variable>();
+            private List<Variable> _outVariableSet = new List<Variable>();
+
+            /// <summary>
+            /// If inVariable has already been copied, return its copy. Otherwise,
+            /// return a fresh Variable associated with inVariable.
+            /// </summary>
+            /// <param name="inVariable"></param>
+            /// <returns></returns>
+            public Variable getCopy(Variable inVariable)
+            {
+                int index = _inVariableSet.IndexOf(inVariable);
+                if (index >= 0)
+                    return _outVariableSet[index];
+                else
+                {
+                    Variable outVariable = new Variable();
+                    _inVariableSet.Add(inVariable);
+                    _outVariableSet.Add(outVariable);
+                    return outVariable;
+                }
+            }
+
+            /// <summary>
+            /// Return the number of unique variables that have been copied.
+            /// </summary>
+            /// <returns></returns>
+            public int getNUniqueVariables()
+            {
+                return _inVariableSet.Count;
+            }
+        }
+    }
+}