Overriding constraints

Using the example below, how can I override the constraints of classes A and B on an instance by instance basis?

For instance, let's say I wanted a1's constraint of VarA0 to be VarA0 > 3 and b1's constraint of  VarB1 to be VarB1

Thanks!


module test();
  class A;
    rand bit [3:0] varA0;
    rand bit [3:0] varA1;

    constraint c0 { varA0 > 1; }
    constraint c1 { varA1 > 2; }
  endclass

  class B;
   rand bit [3:0] varB0;
   rand bit [3:0] varB1;

   constraint c0 { varB0 > 1; }
   constraint c1 { varB1 > 2; }
 endclass

  A a0=new;
  A a1=new;
  B b0=new;
  B b1=new;

initial begin
  if (a0.randomize()==0) begin
    $display("error!");
  end
  if (a1.randomize()==0) begin
    $display("error!");
  end
  if (b0.randomize()==0) begin
    $display("error!");
  end
  if (b1.randomize()==0) begin
    $display("error!");
  end
  $display("a0.varA0=%d",a0.varA0);
  $display("a0.varA1=%d",a0.varA1);
  $display("a1.varA0=%d",a1.varA0);
  $display("a1.varA1=%d",a1.varA1);
  $display("b0.varB0=%d",b0.varB0);
  $display("b0.varB1=%d",b0.varB1);
  $display("b1.varB0=%d",b1.varB0);
  $display("b1.varB1=%d",b1.varB1);
end   
 
endmodule