test-yaml-overrides.rb

#! /bin/env ruby

=begin
Usage: ruby test-yaml-override.rb
Output:
  ---
  letters:
    d: 4
  lettersOne:
    a: 1
    b: 2
    c: 3
=end

require 'yaml'
puts YAML::load(DATA).to_yaml

__END__
letters: &lettersRef
  a: 1
  b: 2

lettersOne:
  <<: *lettersRef
  c: 3

# obliterates all of letters, though lettersOne is unaffected
letters: { d: 4 }