about_arrays.rb

a doubt

arr = [1, 2, 3, 4]

puts "arr[3]: #{arr[3].inspect}"
puts "arr[4]: #{arr[4].inspect}"
puts "arr[5]: #{arr[5].inspect}"

puts "arr[3,0]: #{arr[3,0].inspect}"
puts "arr[4,0]: #{arr[4,0].inspect}" # why is this [] 
puts "arr[5,0]: #{arr[5,0].inspect}" # but this is nil

__END__

ruby 1.9.2p180 (2011-02-18 revision 30909) [x86_64-darwin10.6.0]
arr[3]: 4
arr[4]: nil
arr[5]: nil
arr[3,0]: []
arr[4,0]: []
arr[5,0]: nil

molgrew says that it is a fence variable
checked array.c line 948