reserve x for set;
reserve i,j for Integer;
reserve n,n1,n2,n3 for Nat;
reserve K,K1,K2,K3 for Field;
reserve SK1,SK2 for Subfield of K;
reserve ek,ek1,ek2 for Element of K;
reserve p for Prime;
reserve a,b,c for Element of GF(p);
reserve F for FinSequence of GF(p);

theorem Th13:
  ex n1 st a = n1 mod p
  proof
    reconsider a as Element of Segm(p);
    0<=a & a<p by NAT_1:44;
    then a = a mod p by NAT_D:63;
    hence thesis;
  end;
