DepthFirst

Taking the tree as example, the resulting sequence contains the result in a depth first manner, i.e. if dropRoots = false then the first element of the original sequence is returned first (if dropRoots] = true then this element is skipped/dropped). Next the first child of this first element is returned, then the first child of the first child etc. Once there is no first child (i.e. we reached a leaf of the tree) the sibling of this child is visited, and again the first child of this sibling etc.