code: def save_to_string(self):
      if self.model is None:
          raise ValueError("Cannot serialize model before fitting")
      thundersvm.save_to_string_scikit.restype = c_void_p
      sp = thundersvm.save_to_string_scikit(c_void_p(self.model))
      retval = string_at(sp)
      thundersvm.free_string(cast(sp, c_void_p))
      return retval
  
imports: from sklearn.base import BaseEstimator
  from sklearn.base import RegressorMixin, ClassifierMixin
  import numpy
  import scipy.sparse
  from sklearn.utils import check_X_y, column_or_1d, check_array
  from sklearn.utils.validation import _num_samples
  from ctypes import *
  from os import path, curdir
  from sys import platform
label_desc: Use image node_id 15 for calling the SvmModel obj's underlying member method code with example usage: obj.save_to_string() and returns: retval