code: def subexpr(self):
      localctx = LaTeXParser.SubexprContext(self, self._ctx, self.state)
      self.enterRule(localctx, 74, self.RULE_subexpr)
      try:
          self.enterOuterAlt(localctx, 1)
          self.state = 495
          self.match(LaTeXParser.UNDERSCORE)
          self.state = 501
          self._errHandler.sync(self)
          token = self._input.LA(1)
          if token in [27, 29, 30, 68, 69, 70, 71, 72, 76, 77, 78, 91]:
              self.state = 496
              self.atom()
              pass
          elif token in [21]:
              self.state = 497
              self.match(LaTeXParser.L_BRACE)
              self.state = 498
              self.expr()
              self.state = 499
              self.match(LaTeXParser.R_BRACE)
              pass
          else:
              raise NoViableAltException(self)
  
      except RecognitionException as re:
          localctx.exception = re
          self._errHandler.reportError(self, re)
          self._errHandler.recover(self, re)
      finally:
          self.exitRule()
      return localctx
  
imports: from antlr4 import *
  from io import StringIO
  import sys
label_desc: Use image node_id 39 for calling the LaTeXParser obj's underlying member method code with example usage: obj.subexpr() and returns: localctx