_processTree(ast) {
    let config = this.config;

    this.plugins.filter(function(plugin) {
      return plugin.value !== null &&
             typeof plugin.process === 'function' &&
             plugin.syntax.indexOf(ast.syntax) !== -1;
    }).forEach(function(plugin) {
      plugin.process(ast, config);
    });

    return ast;
  }
@param {Node} ast
@return {Node} Transformed AST