    element.toBack([opt])

Move the element so it is behind all other cells (elements/links). If `opt.deep` is `true`, all the embedded cells of this element will get higher `z` index than that of this element in a Breadth-first search (BFS) fashion. This is especially useful in hierarchical diagrams where if you want to send an element to the back, you don't want its children (embedded cells) to be hidden behind that element.