NSArray *ranges = [view selectedRanges];
for (id range in ranges) {
    NSFont *oldFont = [[view textStorage] attribute:NSFontAttributeName atIndex:[range rangeValue].location effectiveRange:nil];
    NSFont *newFont;
    if([[oldFont fontDescriptor] symbolicTraits] & NSFontBoldTrait)
        newFont = [[NSFontManager sharedFontManager] convertFont:oldFont toNotHaveTrait:NSBoldFontMask];
    else
        newFont = [[NSFontManager sharedFontManager] convertFont:oldFont toHaveTrait:NSBoldFontMask];
    [[view textStorage] addAttributes:[NSDictionary dictionaryWithObject:newFont forKey:NSFontAttributeName] range:[range rangeValue]];
}