to solve differential equations in verilog-a?

Hi,
   Now I'm using verilog-a to do some research on modeling kinematic
structures. We all know that ddt or idt is accepted in verilog-a, such
like the following sentences:
   Pos(v)<+ddt(x);

  But if Fdrv is from a source outside my module and i want to output
the x in the following differential equation, how can i?

  a*x''+b*x'+cx=Fdrv

   i've written these codes,
   .......(includings)
   module test(Fdrv,x);
       inout Fdrv,x;
       kinematic Fdrv,x;
       kinematic v; //temp. var. to hold ddt(x)
       parameter real a=xxx, b=xxx, c=xxx; // assume a,b,c are
properly defined
               analog begin
                    Pos(v)<+ddt(x);
                    F(Fdrv)<+a*ddt(Pos(v))
                                  +b*Pos(v)
                                  +c*Pos(x);
                end
   endmodule

    if i input x to module test, i will get proper output of Fdrv, but
if i want to input Fdrv and get the
proper value of x, which means i want to solve this differential
equation, it never works.
    How can i solve this problem? thank you very very much....