code: def p_reference_variable_string_offset(p):
      'reference_variable : reference_variable LBRACE expr RBRACE'
      p[0] = ast.StringOffset(p[1], p[3], lineno=p.lineno(2)f p_reference_variable_string_offset(p):
      'reference_variable : reference_variable LBRACE expr RBRACE'
      p[0] = ast.StringOffset(p[1], p[3], lineno=p.lineno(2))
docs: reference_variable : reference_variable LBRACE expr RBRACE