code: function walk(node, cb) {
    nast.walk(node,
      function (node) {
        cb(node);
      },
      function (node) {
        cb({type: 'end', node: node});
      }
    );
  }
docs: /*
  Walk AST, calling end callback with pseudo-node.
  @param {AstNode} node Node of extended AST.
  @param {function} cb Function to call at start and end of node.