问QList::push_back()的错误调用

EN

parser.y:40: error: no matching function for call to 'QList<QList<QString> >::insert(QList<QList<QString> >&)'

formula.push_back(partialFormula);

QList<QList<QString> > formula;
QList<QList<QString> > partialFormula;
formula.push_back(partialFormula); //error.

#include <QList>
#include <QString>

int main () {
    QList<QList<QString> > formula;
    QList<QList<QString> > partialFormula;
    QList<QString> atomicCondition;

    //It works!
    partialFormula.push_back(atomicCondition);
    //It does not work.
    formula.push_back(partialFormula);

    return 0;
}