api.js

var lang = require('cssauron')({
      tag: function(node) { return node.type }
    , contents: function(node) { return node.data }
    , id: function(node) { return 'line_'+node.token.line }
    , attr: function(node) { return node[attr] }
    , parent: function(node) { return node.parent }
    , children: function(node) { return node.children }
    , class: function(node) { throw new Error('classes not supported') }
  })

var selector = lang('stmtlist > stmt > decl')

selector(some_ast_node) // true or false