-- Most edge types/labels are unique; a node can have only one edge with each type.

-- Most link types/labels are unique; a node can have only one link with each type.

-- The opposite of a unique edge (e.g. parent) might be non-unique (e.g. child).
Non_unique_edges = {'side', 'child', 'cross', 'before', 'after'}

-- The opposite of a unique link (e.g. parent) might be non-unique (e.g. child).
Non_unique_links = {'side', 'child', 'cross', 'before', 'after'}

  if Links[pane.id][rel] and not array.find(Non_unique_edges, rel) then

  if Links[pane.id][rel] and not array.find(Non_unique_links, rel) then

  add_edge(pane.id, rel, new_pane.id)
  add_edge(new_pane.id, Opposite[rel], pane.id)
  if #Surface[Cursor_pane.col] == 1 and not array.find(Non_unique_edges, rel) then

  add_link(pane.id, rel, new_pane.id)
  add_link(new_pane.id, Opposite[rel], pane.id)
  if #Surface[Cursor_pane.col] == 1 and not array.find(Non_unique_links, rel) then

  if not can_add_edge(pane.id, rel, Grab_pane.id) then

  if not can_add_link(pane.id, rel, Grab_pane.id) then

  if not can_add_edge(Grab_pane.id, Opposite[rel], pane.id) then

  if not can_add_link(Grab_pane.id, Opposite[rel], pane.id) then

  add_edge(pane.id, rel, Grab_pane.id)
  add_edge(Grab_pane.id, Opposite[rel], pane.id)

  add_link(pane.id, rel, Grab_pane.id)
  add_link(Grab_pane.id, Opposite[rel], pane.id)

-- Can we add an edge called 'rel' from src to target?
function can_add_edge(src, rel, target)

-- Can we add a link called 'rel' from src to target?
function can_add_link(src, rel, target)

  if array.find(Non_unique_edges, rel) then

  if array.find(Non_unique_links, rel) then

function add_edge(src, rel, target)
  if array.find(Non_unique_edges, rel) then

function add_link(src, rel, target)
  if array.find(Non_unique_links, rel) then

function remove_edge(src, rel, target)

function remove_link(src, rel, target)

  if array.find(Non_unique_edges, rel) then

  if array.find(Non_unique_links, rel) then

  if array.find(Non_unique_edges, from) then

  if array.find(Non_unique_links, from) then

  if Links[pane.id][to] and not array.find(Non_unique_edges, to) then

  if Links[pane.id][to] and not array.find(Non_unique_links, to) then

  remove_edge(pane.id, from, target)
  add_edge(pane.id, to, target)

  remove_link(pane.id, from, target)
  add_link(pane.id, to, target)

    remove_edge(target, Opposite[from], pane.id)
    add_edge(target, Opposite[to], pane.id)

    remove_link(target, Opposite[from], pane.id)
    add_link(target, Opposite[to], pane.id)

  if array.find(Non_unique_edges, rel) then

  if array.find(Non_unique_links, rel) then

  if array.find(Non_unique_edges, rel) then

  if array.find(Non_unique_links, rel) then

  if array.find(Non_unique_edges, rel) then

  if array.find(Non_unique_links, rel) then