nl_tokens: RoseTree a - > ( a - > Bool ) - > RoseTree a
pl_tokens: function filterSubtrees ( pred , tree ) { return new RoseTree ( tree . root , function ( ) { return tree . <mask> ( ) . filter ( function ( subtree ) { return pred ( subtree . root ) ; } ) . map ( filterSubtrees . bind ( null , pred ) ) ; } ) ; }