reserve X for set;
reserve a,b,c,k,m,n for Nat;
reserve i for Integer;
reserve r for Real;
reserve p for Prime;

theorem Th42:
  5 divides 2|^645-2
  proof
    (2|^4)|^161,1|^161 are_congruent_mod 5 by Lm1141,GR_CY_3:34;
    then 2|^644*2|^1,1*2 are_congruent_mod 5 by Lm1124,INT_4:11;
    hence thesis by Lm1126;
  end;
