void fp_set(fp_int *a, fp_digit b)
{
   fp_zero(a);
   a->dp[0] = b;
   a->used  = a->dp[0] ? 1 : 0;
}
void fp_set(fp_int *a, fp_digit b)
{
   fp_zero(a);
   a->dp[0] = b;
   a->used  = a->dp[0] ? 1 : 0;
}