class Example
  def initialize(@this_AT_in_the_begining_is_something_Ruby_chokes)
  end
end


=begin

Outputs with my old Ruby:

parse_example_1.rb:2: parse error
  def initialize(@this_AT_in_the_begining_is_something_Ruby_chokes)
                                                                  ^


Maybe one can use this as a patch. (Took against my old Ruby.)

--- parse.y     Thu Apr 13 10:17:20 2000
+++ parse.y.2   Fri Jun 30 01:36:20 2000
@@ -1583,9 +1583,21 @@
                        $$ = NEW_ARGS(0, 0, -1);
                    }

-f_norm_arg     : tCONSTANT
+f_norm_arg     : tCONSTANT
                    {
-                       yyerror("formal argument must not be constant");
+                       yyerror("formal argument cannot be a constant");
+                   }
+                | tIVAR
+                   {
+                       yyerror("formal argument cannot be an instance
variable");
+                   }
+                | tGVAR
+                   {
+                       yyerror("formal argument cannot be a global
variable");
+                   }
+                | tCVAR
+                   {
+                       yyerror("formal argument cannot be a class
variable");
                    }
                | tIDENTIFIER
                    {

Outputs now:

parse_example_1.rb:2: formal argument cannot be an instance variable
  def initialize(@this_AT_in_the_begining_is_something_Ruby_chokes)
                                                                  ^


=end