Code -->
rand bit ⎛:0] x,y,a,b;
covergroup cg_xy @(posedge clock);
  X  : coverpoint x;
  Y  : coverpoint y;
  A  : coverpoint a;
  B  : coverpoint b;
  XY : cross X, Y;
  AB : cross A, B;
  XA : cross XY, AB;  // IS THIS ALLOWED ??
endgroup

Simulator -->
QuestaSim  vsim 6.5c Simulator 2009.08 Aug 27 2009