code: function getPlaceholder() {
    const type = NodeType.PlaceholderType;
    const token = tokens[pos];
    const line = token.ln;
    const column = token.col;
    let content = [];
  
    // Skip `%`.
    pos++;
  
    content = content.concat(getIdentOrInterpolation());
  
    return newNode(type, content, line, column);
  }
docs: Get node with a placeholder selector
  @return {Array} `['placeholder', ['ident', x]]` where x is a placeholder's
  identifier (without `%`, e.g. `abc`).