public FunctionExpression(string expr, Context context)
     : base(expr, context)
 {
     _args = new List<AbstractExpression>();
 }
 public Interpreter()
 {
     _context = new Context();
 }
 public AbstractExpression(string expr, Context context)
 {
     _expr = expr;
     _context = context;
 }
 public IntegerExpression(string expr, Context context)
     : base(expr, context)
 {
 }