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$ /usr/local/bin/irb
irb(main):001:0> require 'rubygems'
=> true
irb(main):002:0> require 'postgres'
LoadError: no such file to load -- postgres
        from /usr/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `gem_original_require'
        from /usr/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `require'
        from (irb):2
irb(main):003:0>