/usr/lib/ruby/2.5.0/rdoc/extend.rb is in libruby2.5 2.5.1-1ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 | # frozen_string_literal: true
##
# A Module extension to a class with \#extend
#
# RDoc::Extend.new 'Enumerable', 'comment ...'
class RDoc::Extend < RDoc::Mixin
end
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