reserve a,b,c,k,m,n for Nat;
reserve i,j,x,y for Integer;
reserve p,q for Prime;
reserve r,s for Real;

theorem Th5:
  -1 mod p = p-1
  proof
    p-1 < p-0 by XREAL_1:8;
    then p-1 = (p-1) mod p by NAT_D:24
    .= ((p-1) + (-1)*p) mod p by NAT_D:61;
    hence thesis;
  end;
