add method
Implementation
void add(NodeOutput item) {
if (item.anchorMargin == AnchorMargin.end) {
if (end != null) {
throw "end is occupied";
}
end = item;
}
if (item.anchorMargin == AnchorMargin.start) {
if (start != null) {
throw "start is occupied";
}
start = item;
}
}