theorem Th87:
  not 7 divides a|^3 - 2 & not 7 divides a|^3 + 2 &
    not 7 divides a|^3 - 3 & not 7 divides a|^3 + 3
  & not 7 divides a|^3 - 4 & not 7 divides a|^3 + 4 &
    not 7 divides a|^3 - 5 & not 7 divides a|^3 + 5
  proof
    7 = 2*3 + 1;
    hence thesis by Th84,NAT_4:26;
  end;
