class Psych::Visitors::YAMLTree

def visit_Psych_Set o
  register(o, @emitter.start_mapping(nil, '!set', false, Psych::Nodes::Mapping::BLOCK))
  o.each do |k,v|
    accept(@stringify_names && Symbol === k ? k.to_s : k)
    accept v
  end
  @emitter.end_mapping
end