EFS file.setPosition() issue

[bos-fx]

I try to make study which will write files, and after writing string to file, put cariiage to the fixed place and put string began from this place.

function preMain() {
setPriceStudy(true);
setShowCursorLabel(false);
}


function main()
{
var a = new File("temp.csv");
var b = new File("SLW3-SLV3.txt");

b.open("rt");
b.getLength();

a.open("at+");
a.write("Hello world! carriage:");
a.writeln(a.getPosition()+"and"+b.getLength());
a.setPosition(150);
a.write("NewPosition:"+a.getPosition());
a.close();
b.close();


}

But I see, that setPosition() gets only 0! And it's anyway write the string "NewPosition:" not begining from 0! How can I solve it?