Unknow algoritm in code

Advanced Topics Development
1

in block fixlayout

line.forEach(function (part) {
if (part.isLoop) {
hasLoopCSlot = true;
}
if (part instanceof CSlotMorph) {
...
} else if (part instanceof MultiArgMorph &&
(part.slotSpec === '%cs')) {
...
} else {
part.setPosition(new Point(x, y));
if (!part.isBlockLabelBreak) {
if (part.slotSpec === '%c' || part.slotSpec === '%loop') {
x += part.width();(impassible commands)
} else if (part.isVisible) {
...
}
}
...
);
}

%c make CSlotMorph in labelPart function and i think that
if part.slotSpec exist then part is instanceof CSlotMorph
i think that maybe we have syntax mistake here