code: def eval_at_sub(self):
      localctx = LaTeXParser.Eval_at_subContext(
          self, self._ctx, self.state
      )
      self.enterRule(localctx, 26, self.RULE_eval_at_sub)
      try:
          self.enterOuterAlt(localctx, 1)
          self.state = 180
          self.match(LaTeXParser.UNDERSCORE)
          self.state = 181
          self.match(LaTeXParser.L_BRACE)
          self.state = 184
          self._errHandler.sync(self)
          la_ = self._interp.adaptivePredict(self._input, 12, self._ctx)
          if la_ == 1:
              self.state = 182
              self.expr()
              pass
  
          elif la_ == 2:
              self.state = 183
              self.equality()
              pass
  
          self.state = 186
          self.match(LaTeXParser.R_BRACE)
      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 15 for calling the LaTeXParser obj's underlying member method code with example usage: obj.eval_at_sub() and returns: localctx