package tree;
      has '_branches' => (
               traits    =>  ['Hash'], 
               is        => 'rw', 
               isa       => 'HashRef[Any]', 
               handles   => {
                  _set_branch  => 'set',
                  _is_branch   => 'defined',
                  _list_branches => 'keys',
                  _branch => 'get'
               },
               trigger   => sub {
                  my($self,$hash) = @_;
                  $self->_build_branch($hash);
               }

      );


sub _build_branch{
  my($self,$hash);
  # do stuff!
  #return altered or coerced hash
  return $hash;
}