docs: def p_compound_variable(p):
      '''compound_variable : VARIABLE
                           | DOLLAR LBRACE expr RBRACE'''
      if len(p) == 2:
          p[0] = ast.Variable(p[1], lineno=p.lineno(1))
      else:
          p[0] = ast.Variable(p[3], lineno=p.lineno(1))