What does "compatible for Ruby 1.9.1" mean ?

In phosphorescence: The 2nd day of RubyKaigi 2009 (Morning), I mentioned about "Ruby 1.9.x is compatible for Ruby 1.9.1" and announced to describe it after. So I describe about it.

It's simple, but outlandish rule. For example in 1.8.x(1.8.7),

> ruby -e 'puts RUBY_VERSION, $:'
1.8.7
/usr/lib64/ruby/site_ruby/1.8
/usr/lib64/ruby/site_ruby/1.8/x86_64-linux
/usr/lib64/ruby/site_ruby
/usr/lib64/ruby/vendor_ruby/1.8
/usr/lib64/ruby/vendor_ruby/1.8/x86_64-linux
/usr/lib64/ruby/vendor_ruby
/usr/lib64/ruby/1.8
/usr/lib64/ruby/1.8/x86_64-linux
.

But for example in 1.9.x(1.9.2),

> /opt/ruby-1.9.1/bin/ruby -e 'puts RUBY_VERSION, $:'
1.9.2
/opt/ruby-1.9.1/lib/ruby/gems/1.9.1/gems/rake-0.8.7/bin
/opt/ruby-1.9.1/lib/ruby/gems/1.9.1/gems/rake-0.8.7/lib
/opt/ruby-1.9.1/lib/ruby/gems/1.9.1/gems/rdoc-2.4.3/bin
/opt/ruby-1.9.1/lib/ruby/gems/1.9.1/gems/rdoc-2.4.3/lib
/opt/ruby-1.9.1/lib/ruby/gems/1.9.1/gems/rubygems-update-1.3.5/bin
/opt/ruby-1.9.1/lib/ruby/gems/1.9.1/gems/rubygems-update-1.3.5/lib
/opt/ruby-1.9.1/lib/ruby/site_ruby/1.9.1
/opt/ruby-1.9.1/lib/ruby/site_ruby/1.9.1/x86_64-linux
/opt/ruby-1.9.1/lib/ruby/site_ruby
/opt/ruby-1.9.1/lib/ruby/vendor_ruby/1.9.1
/opt/ruby-1.9.1/lib/ruby/vendor_ruby/1.9.1/x86_64-linux
/opt/ruby-1.9.1/lib/ruby/vendor_ruby
/opt/ruby-1.9.1/lib/ruby/1.9.1
/opt/ruby-1.9.1/lib/ruby/1.9.1/x86_64-linux

We need imprinting "such a thing" to ourselves. But more confusion from late majorities will occur perhaps when first stable 1.9.2 will be released.