theorem Th74:
  a,b are_conjugated iff ex g st b = a * g
proof
  thus a,b are_conjugated implies ex g st b = a * g
  proof
    given g such that
A1: a = b * g;
    a * (-g) = b by A1,ThB25;
    hence thesis;
  end;
  given g such that
A2: b = a * g;
  a = b * (-g) by A2,ThB25;
  hence thesis;
end;
