code: def _impl(lexer):
      
      p = _sumterm(lexer)
  
      tok = next(lexer)
      # SUMTERM '=>' IMPL
      if isinstance(tok, OP_rarrow):
          q = _impl(lexer)
          return ('implies', p, q)
      # SUMTERM '<=>' IMPL
      elif isinstance(tok, OP_lrarrow):
          q = _impl(lexer)
          return ('equal', p, q)
      # SUMTERM
      else:
          lexer.unpop_token(tok)
          return p
docs: Return an Implies expression.