void  add_poly(JD *pa,JD *pb)
{  JD *p,*q,*u,*pre;
   int x;
   p=pa->next;
   q=pb->next;
   pre=pa;
   while((p!=NULL) && ((q!=NULL))
   {   if(p->exp<q->exp)
       {   pre=p;  p=p->next;}
       else if(p->exp==q->exp)
       {  x=p->coef+q->coef;
          if(x!=0){ p->coef=x;  pre=p;}
          else    { pre->next=p->next;  free(p);}
          p=pre->next;
          u=q;
          q=q->next;
          free(u);
       }
       else
       {    u=q->next;q->next=p;pre->next=q;
            pre=q;  q=u;
       }
    }
    if(q!=NULL)
        pre->next=q;
    free(pb);
}