not able to find to_yaml definition

I am looking at rails edge source code.

I am able to do

puts User.find.to_yaml

However I am not able to find piece of code where 'def to_yaml' is.
How this to_yaml serialization is working?

can fully understand how to_json and to_xml is working but to_yaml
beats me.

Thanks

YAML is part of Ruby. Check out http://www.ruby-doc.org/core/ for
YAML class readme.

rick