c:\>irb
irb(main):001:0> Fixnum.class_eval do
irb(main):002:1*   original_add = instance_method(:+)
irb(main):003:1>   define_method(:+) do |other|
irb(main):004:2*     if other.kind_of?(String)
irb(main):005:3>       return String(self) + other
irb(main):006:3>     else
irb(main):007:3*       return original_add.bind(self).call(other)
irb(main):008:3>     end
irb(main):009:2>   end
irb(main):010:1> end
=> #<Proc:0x043db374@(irb):3>
irb(main):011:0>
irb(main):012:0*
irb(main):013:0*
irb(main):014:0* String.class_eval do
irb(main):015:1*   original_add = instance_method(:+)
irb(main):016:1>   define_method(:+) do |other|
irb(main):017:2*     if other.kind_of?(Fixnum)
irb(main):018:3>       return self + String(other)
irb(main):019:3>     else
irb(main):020:3*       return original_add.bind(self).call(other)
irb(main):021:3>     end
irb(main):022:2>   end
irb(main):023:1> end
=> #<Proc:0x043a6994@(irb):16>
irb(main):024:0> '1' + 1
=> "11"
irb(main):025:0> 1 + '1'
=> "11"
irb(main):026:0> 1 + 1
=> 2
irb(main):027:0> '1' + '1'
=> "11"
irb(main):028:0>