/**
* Choose a unique tab file.
*
* @return io reference
*/
private IOFile newTabFile() {
// collect numbers of existing files
final BoolList bl = new BoolList();
for (final EditorArea edit : editors()) {
if (edit.opened()) continue;
final String n = edit.file.name().substring(FILE.length());
bl.set(n.isEmpty() ? 1 : Integer.parseInt(n), true);
}
// find first free file number
int c = 0;
while (++c < bl.size() && bl.get(c)) ;
// create io reference
return new IOFile(gui.gprop.get(GUIProp.WORKPATH), FILE + (c == 1 ? "" : c));
}