irb(main):001:0> class Fixnum
irb(main):002:1>   def + other
irb(main):003:2>     self - other
irb(main):004:2>   end
irb(main):005:1> end
=> nil
irb(main):006:0>
irb(main):005:0*
irb(main):004:0*
irb(main):003:0*
irb(main):002:0*
irb(main):001:0*
irb(main):000:0*
irb(main):-01:0*
irb(main):-02:0*
irb(main):-03:0*
irb(main):-04:0*
irb(main):-05:0*
/usr/lib/ruby/1.9.1/irb/input-method.rb:117:in `[]=': index -7 too small for array; minimum: -6 (IndexError)
        from /usr/lib/ruby/1.9.1/irb/input-method.rb:117:in `gets'
        from /usr/lib/ruby/1.9.1/irb.rb:139:in `block (2 levels) in eval_input'
        from /usr/lib/ruby/1.9.1/irb.rb:273:in `signal_status'
        from /usr/lib/ruby/1.9.1/irb.rb:138:in `block in eval_input'
        from /usr/lib/ruby/1.9.1/irb/ruby-lex.rb:188:in `call'
        from /usr/lib/ruby/1.9.1/irb/ruby-lex.rb:188:in `buf_input'
        from /usr/lib/ruby/1.9.1/irb/ruby-lex.rb:103:in `getc'
        from /usr/lib/ruby/1.9.1/irb/slex.rb:205:in `match_io'
        from /usr/lib/ruby/1.9.1/irb/slex.rb:75:in `match'
        from /usr/lib/ruby/1.9.1/irb/ruby-lex.rb:286:in `token'
        from /usr/lib/ruby/1.9.1/irb/ruby-lex.rb:262:in `lex'
        from /usr/lib/ruby/1.9.1/irb/ruby-lex.rb:233:in `block (2 levels) in each_top_level_statement'
        from /usr/lib/ruby/1.9.1/irb/ruby-lex.rb:229:in `loop'
        from /usr/lib/ruby/1.9.1/irb/ruby-lex.rb:229:in `block in each_top_level_statement'
        from /usr/lib/ruby/1.9.1/irb/ruby-lex.rb:228:in `catch'
        from /usr/lib/ruby/1.9.1/irb/ruby-lex.rb:228:in `each_top_level_statement'
        from /usr/lib/ruby/1.9.1/irb.rb:155:in `eval_input'
        from /usr/lib/ruby/1.9.1/irb.rb:70:in `block in start'
        from /usr/lib/ruby/1.9.1/irb.rb:69:in `catch'
        from /usr/lib/ruby/1.9.1/irb.rb:69:in `start'
        from /usr/bin/irb:12:in `<main>'