(
on isEnabled return
(
  selection.count == 1 and \
  (classof selection[1] == EditSpline or\
  classof selection[1] == Line) \
  and selection[1].modifiers.count == 0
)
on execute do
(
  new_z = $.pos.z
  for s = 1 to (numSplines $) do
  (
    for k = 1 to (numKnots $ s) do
    (
      knt = getKnotPoint $ s k
      in_vec = getInVec $ s k
      out_vec = getOutVec $ s k
      knt.z = in_vec.z = out_vec.z = new_z
      setInVec $ s k in_vec
      setOutVec $ s k out_vec
      setKnotPoint $ s k knt
    )--end k loop
  )--end s loop
  updateshape $
)--end execute
)--end script