removeChain method
Implementation
VertexList removeChain(Vertex a, Vertex b ) {
if ( a.prev == null ) {
head = b.next;
} else {
a.prev?.next = b.next;
}
if ( b.next == null ) {
tail = a.prev;
}
else {
b.next?.prev = a.prev;
}
a.prev = null;
b.next = null;
return this;
}