question: PlaPointRational ( BigInteger p_x , BigInteger p_y , BigInteger p_z ) { rp_x = p_x ; rp_y = p_y ; rp_z = p_z ; is_nan = p_z . signum ( ) <= 0 ; }
target:  creates a RetionalPoint from 3 BigIntegers p_x, p_y and p_z.  They represent the 2-dimensinal point with the rational number Tuple ( p_x / p_z , p_y / p_z).  Throws IllegalArgumentException if denominator p_z is <= 0