protected CommandExpression(CommandExpression caller, ExpressionFunction function)
 {
     _functionCaller = caller;
     this.Function   = function;
 }
 internal static CommandExpression FromFunction(ExpressionFunction function)
 {
     return(new CommandExpression(function));
 }
 protected CommandExpression(ExpressionFunction function)
 {
     this.Function = function;
 }