code: function getVariable() {
    const type = NodeType.VariableType;
    const token = tokens[pos];
    const line = token.ln;
    const column = token.col;
    const content = [];
  
    // Skip `$`.
    pos++;
  
    if (checkVariable(pos)) content.push(getVariable());
    else content.push(getIdent());
  
    return newNode(type, content, line, column);
  }
docs: Get node with a variable
  @returns {Array} `['variable', ['ident', x]]` where `x` is
  a variable name.