Constraint a 9x9 matrix

SystemVerilog
,
4 

module tb;
  class my_item;
 
  rand bit a[9][9];
 
    constraint c{a.sum() with ($countones(item)) ==10;}
 
 
  
endclass
  
  initial begin
    my_item m1;
    
    m1=new();
    assert(m1.randomize());
  $display ("%p",m1.a);
  end
  
endmodule