code: def p_expr_id_substr(p):
      
      entry = SYMBOL_TABLE.access_var(p[1], p.lineno(1), default_type=TYPE.string)
      p[0] = None
      if entry is None:
          return
  
      entry.accessed = True
      p[0] = make_strslice(p.lineno(1), entry, p[2][0], p[2][1])
docs: string : ID substr