The outer cross validation was repeated ten times with tenfold cross validation to validate the radiomics RF classifier, and the inner cross-validation was repeated three times with tenfold cross validation for recursive feature elimination and training of the
radiomics-based classifier 。就是外部用10折交叉验证重复10次，内部用10折交叉验证重复3次验证；这个nested  cross-validation 在R语言中SVM及RFE 函数中是如何实现的