Hi all.

I've encountered enigmatic error message while learning rdoc. This is
the simple program which tests RDoc:

#!/usr/local/bin/ruby -w

# == Synopsis
# Test synopsis

require "optparse"
require "rdoc/usage"

opts = OptionParser.new
opts.on("-h", "--help") { RDoc::usage }
opts.parse(ARGV) rescue RDoc::usage('usage')

puts "hello"


When I run above program like './test.rb', ruby says
"/usr/local/lib/ruby/1.8/rdoc/ri/ri_formatter.rb:46: warning: (...)
interpreted as grouped expression"

Why is this warning raised? I have no idea what grouped expression is.

Sincerely,
Minkoo Seo