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