lib/RoseTree.js
function fmap(f, tree) {
  return new RoseTree(
    f(tree.root),
    function() {
      return tree.children().map(fmap.bind(null, f));
    }
  );
}
function fmap(f, tree) {
  return new RoseTree(
    f(tree.root),
    function() {
      return tree.children().map(fmap.bind(null, f));
    }
  );
}
function fmap ( f , tree ) { return new RoseTree ( f ( tree . root ) , function ( ) { return tree . children ( ) . map ( fmap . bind ( null , f ) ) ; } ) ; }
(a -> b) -> RoseTree a -> RoseTree b
( a - > b ) - > RoseTree a - > RoseTree b