Question: What causes that error?

Hi all, 

I'm trying to implement the smithChart on Maple, but this error "Error, (in unknown) invalid transform: output is not 2 or 3 element" keeps popping up for some reason. I have no idea how to fix it. Can anyone help me with this? Here is the smithChart:




smithChart.mw

smithChart := proc(r)
  local i, a, b, c;
   a := PLOT(seq(plottools[arc]([-i*r/4, 0], i*r/4, 0..Pi),
                 i = 1..4),

  plottools[arc]([0, r/2], r/2,
            Pi-arcsin(3/5)..3*Pi/2),
  plottools[arc]([0,r], r, Pi..Pi+arcsin(15/17)),
  plottools[arc]([0, 2*r], 2*r,
            Pi+arcsin(3/5)..Pi+arcsin(63/65)),
  plottools[arc]([0,4*r], 4*r,
            Pi+arcsin(15/17)..Pi+arcsin(63/65)));

  b := plottools[transform]((x, y) -> [x-y])(a);
  c := plottools[line]([0,0], [-2*r, 0]):
  plots[display](a, b, c, axes = none, scaling = constrained, args[2..nargs]);
  end proc:

smithChart(1);
Error, (in unknown) invalid transform: output is not 2 or 3 elements