% ruby --version
ruby 1.8.3 (2005-09-21) [i686-linux]

% cat a.rb
require 'yaml'

o = [ 'array', 'of', 'items' ]
o2 = YAML::load( o.to_yaml )

% ruby -w a.rb
(eval):2: warning: method redefined; discarding old taguri
(eval):2: warning: method redefined; discarding old taguri
(eval):2: warning: method redefined; discarding old taguri
(eval):14: warning: redefine yaml_tag_subclasses?
(eval):2: warning: method redefined; discarding old taguri
(eval):2: warning: method redefined; discarding old taguri
(eval):2: warning: method redefined; discarding old taguri
(eval):14: warning: redefine yaml_tag_subclasses?
(eval):2: warning: method redefined; discarding old taguri
(eval):2: warning: method redefined; discarding old taguri
(eval):2: warning: method redefined; discarding old taguri
(eval):14: warning: redefine yaml_tag_subclasses?
(eval):2: warning: method redefined; discarding old taguri
(eval):14: warning: redefine yaml_tag_subclasses?
(eval):2: warning: method redefined; discarding old taguri
(eval):2: warning: method redefined; discarding old taguri
(eval):14: warning: redefine yaml_tag_subclasses?
(eval):2: warning: method redefined; discarding old taguri
(eval):2: warning: method redefined; discarding old taguri
(eval):2: warning: method redefined; discarding old taguri
(eval):2: warning: method redefined; discarding old taguri
(eval):14: warning: redefine yaml_tag_subclasses?
(eval):2: warning: method redefined; discarding old taguri
(eval):2: warning: method redefined; discarding old taguri
(eval):2: warning: method redefined; discarding old taguri
(eval):2: warning: method redefined; discarding old taguri
(eval):2: warning: method redefined; discarding old taguri
(eval):2: warning: method redefined; discarding old taguri
/usr/lib/ruby/1.8/yaml/types.rb:13: warning: method redefined; discarding old 
initialize
/usr/lib/ruby/1.8/yaml/types.rb:28: warning: method redefined; discarding old 
initialize
(eval):2: warning: method redefined; discarding old taguri
(eval):2: warning: method redefined; discarding old taguri
(eval):2: warning: method redefined; discarding old taguri
(eval):6: warning: instance variable @taguri not initialized
(eval):6: warning: instance variable @taguri not initialized
(eval):6: warning: instance variable @taguri not initialized
(eval):6: warning: instance variable @taguri not initialized
/usr/lib/ruby/1.8/yaml.rb:133: warning: instance variable @input not 
initialized