QTextTable::insertColumns() don't work

Hi, why insertColumns() dont'work in this example?

Code:

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#include <QDebug>
#include <QVBoxLayout>
#include <QTextEdit>
#include <QPushButton>
#include <QTextTable>
#include "mainwindow.h"
 
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    QWidget* w = new QWidget(this);
 
    QVBoxLayout* lay = new QVBoxLayout(w);
    edit = new QTextEdit(this);
    lay->addWidget(edit);
    QPushButton* changeTable;
    changeTable = new QPushButton ( trUtf8("Change table") );
    connect( changeTable, SIGNAL( clicked(bool) ), SLOT( SlotChangeTable() ) );
    lay->addWidget(changeTable);
 
    setCentralWidget(w);
 
 
    // addTable
    QTextTableFormat tableFormat;
    tableFormat.setAlignment(Qt::AlignCenter);
    tableFormat.setBorderStyle( QTextTableFormat::BorderStyle_Solid );
    tableFormat.setCellPadding( 4 );
    tableFormat.setCellSpacing( 0 );
    tableFormat.setWidth( QTextLength( QTextLength::PercentageLength, 100 ) );
 
    edit->textCursor().insertTable(3, 7, tableFormat);
 
    QTextCursor cursor(edit->textCursor());
    cursor.setPosition(1);
    edit->setTextCursor(cursor);
 
    QTextTable* table = edit->textCursor().currentTable();
    table->mergeCells( 0, 0, 2 , 1);
 
    table->mergeCells( 0, 1, 2 , 1);
    table->mergeCells( 0, 2, 2 , 1);
    table->mergeCells( 0, 3, 2 , 1);
    table->mergeCells( 0, 5, 2 , 1);
    table->mergeCells( 0, 6, 2 , 1);
    table->mergeCells( 2, 0, 1, 6);
 
}
 
MainWindow::~MainWindow()
{
 
}
 
void MainWindow::SlotChangeTable()
{
    QTextTable* table = edit->textCursor().currentTable();
    qDebug() << table->columns();
    table->insertColumns(1,1);
}

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