/*
Gets node of an AST which path

@param {Node} ast
@param {array} path

@returns {Node}
function getNode(ast, path) {
  if (path.length) {
    if (ast.tag === MAP_TAG) {
      let value = ast.value;
      for (let i = 0; i < value.length; ++i) {
        let [keyNode, valNode] = value[i];
        if (path[0] === keyNode.value) {
          return getNode(valNode, path.slice(1));
        }
      }
      return undefined;
    } else if (ast.tag === SEQ_TAG) {
      return ast.value[path[0]] && getNode(ast.value[path[0]], path.slice(1));
    }
  }
  return ast;
}