trying to work out what is going on with monkey patch of String

Somewhere in all of the ruby code that gets included into this app I
am working on, there is something that defined String so that the
following is a valid statement

puts "contact us at " + ""[] + " and say hello"

The app that runs, is able to include that stuff and Ruby doesn't
complain. When I try to write a test module and
include some of the code, ruby complains beuse my load paths or some
part of the configuration is different. I am not really sure.

If I monkey patch string like below, it seems to stop the errors, but
I am not sure if it could be causing other problems.

class String
  def [](x=nil,b=nil)

I am on windows, but I am using cygwin. I did something like what's
below, but did not find where [] might be defined:

find . -name "*.rb" -exec grep -A 15 "class String" {} \; -print > /

Any other ideas to figure this out ? How can I tell inside irb or
something what the code is for ""[]
or where it is defined ?

You could use ruby-debug (assuming that is available on Windows, I
don't know) and step into "string"[] to see where it is defined.