theorem Th24:
  for g2, a be Element of GF(p) st g2 = 2 mod p holds g2*a - a = a
  proof
    let g2, a be Element of GF(p) such that
    A1: g2 = 2 mod p;
    reconsider g1 = 1 mod p as Element of GF(p) by EC_PF_1:14;
    A2: g1 = (2-1) mod p;
    p > 1 by INT_2:def 4;
    then g1 = 1 by NAT_D:63
    .= 1.GF(p) by EC_PF_1:12;
    then g2*a - (1.GF(p))*a = (1.GF(p))*a by A1,A2,Th21;
    hence thesis;
  end;
