public void DoUndo(IGraphProcessingEnvironment procEnv)
 {
     // nothing to do, this is only needed to distinguish the outermost transaction
     // from the nested transactions to know when to remove the rollback information
     // if nothing happened from opening the first transaction to opening the transaction of interest,
     // they would be indistinguishable as the number of undo items did not change
 }

        // gets the variable value, decides whether to query the graph-global or the sequence-lokal variables
        public object GetVariableValue(IGraphProcessingEnvironment procEnv)
        {
            if(Type == "") {
                return procEnv.GetVariableValue(name);
            } else {
#if LOG_VARIABLE_OPERATIONS
                procEnv.Recorder.Write(name + "==" + EmitHelper.ToStringAutomatic(value, procEnv.Graph) + "\n");
#endif
                return value;
            }
        }

        public SequenceVariable Copy(Dictionary<SequenceVariable, SequenceVariable> originalToCopy, IGraphProcessingEnvironment procEnv)
        {
            // ensure that every instance of a variable is mapped to the same copy
            if(originalToCopy.ContainsKey(this))
                return originalToCopy[this];

            // local variables must be cloned when a defined sequence gets copied
            // global variables stay the same
            if(Type == "")
            {
                originalToCopy.Add(this, this);
                return this;
            }
            else
            {
                originalToCopy.Add(this, new SequenceVariable(name, prefix, type));
#if LOG_VARIABLE_OPERATIONS
                procEnv.Recorder.Write(name + " = " + name + "; " + name + "==" + EmitHelper.ToStringAutomatic(value, procEnv.Graph) + "\n");
#endif
                return originalToCopy[this];
            }
        }

 public override object Execute(IGraphProcessingEnvironment procEnv)
 {
     object entity = Entity.Evaluate(procEnv);
     return TypesHelper.XgrsTypeOfConstant(entity, procEnv.Graph.Model);
 }

 internal override SequenceExpression CopyExpression(Dictionary<SequenceVariable, SequenceVariable> originalToCopy, IGraphProcessingEnvironment procEnv)
 {
     SequenceExpressionFunctionMethodCall copy = (SequenceExpressionFunctionMethodCall)MemberwiseClone();
     copy.TargetExpr = TargetExpr.CopyExpression(originalToCopy, procEnv);
     copy.ParamBindings = ParamBindings.Copy(originalToCopy, procEnv);
     return copy;
 }

        // sets the variable value, decides whether to update the graph-global or the sequence-lokal variables
        public void SetVariableValue(object value, IGraphProcessingEnvironment procEnv)
        {
            if(Type == "") {
                procEnv.SetVariableValue(name, value);
            } else {
#if LOG_VARIABLE_OPERATIONS
                procEnv.Recorder.Write(name + " = " + EmitHelper.ToStringAutomatic(value, procEnv.Graph) + "\n");
#endif
                this.value = value;
            }
        }