indent selection in QPlainTextEdit

[qt_gotcha]

I do this to increase the indent of a selection of lines in a plainTextEdit, but nothing happens:

QTextCursor cur = plainEdit->textCursor(); // contains the selection
	QTextBlockFormat tbf = cur.blockFormat(); //selection format
	tbf.setIndent(10); // change indent
	cur.setBlockFormat(tbf);  // put it all back
	plainEdit->setTextCursor(cur);

QTextCursor cur = plainEdit->textCursor(); // contains the selection
	QTextBlockFormat tbf = cur.blockFormat(); //selection format
	tbf.setIndent(10); // change indent
	cur.setBlockFormat(tbf);  // put it all back
	plainEdit->setTextCursor(cur);

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wordwrap is off so a block is a line. This doesn't work, somebody has some pointers? thanks