Question: how to generate the forrtan for this proc?

Hello, recently I am trying to generate fortran codes for the proc below:

VWN5 := proc()
   local rhoA, rhoB, rhoT, xi, x, g_xi, Q, X, eps, ECF, ECA, ECP, fac, fac1, G, D1F_A, D1F_B;
   rhoT := rhoA + rhoB;
      xi   := rhoA - rhoB;
      x    := (3/(4*Pi*rhoT))^(1/6);
   g_xi := (9/8)*( (1+xi)^(4/3) + (1-xi)^(4/3) - 2);
     Q    := (4*c-b^2)^(1/2);
     X    := x -> x^2 + b*x + c;
   eps  := (A, x0, b, c) -> A*(ln(x^2/X(x)) + \
   (2*b/Q)*cot(Q/(2*x+b)) - b*x0/X(x0)* (ln((x-x0)^2/X(x)) + (2*(2*x0+b)/Q)*cot(Q/2*x+b)));
   ECF  := eps (0.01554535000,  0.32500,  7.06042, 18.0578 );
   ECP  := eps (0.03109070000,  0.10498,  3.72744, 12.9352);
   ECA  := eps (-1/(6*Pi^(-2)), 0.228344, 1.06835, 11.4813);
   fac  := 1 + (4/(9*(2^(1/3)-1)) * (ECF-ECP)/ECA -1)*xi^4;
   fac1 := ECP + ECA*g_xi*fac;
   G    := rhoT*fac1;
   D1F_A:= diff (functional, rhoA);
   D1F_B:= diff (functional, rhoB);
end proc:

After I read in the VWN5 proc , and I typed:

with(codegen);

then I wish to transform the above proc into the fortran codes, so I typed:

fortran (VWN5);

it says that:

Error, (in codegen/fortran) unable to translate lex[1]

So how can I solve this problem? Thanks in advance.