def _erfc(a):
    """
    Port of cephes ``ndtr.c`` ``erfc`` function.

    See https://github.com/jeremybarnes/cephes/blob/master/cprob/ndtr.c
    """
    # approximation for abs(a) < 8 and abs(a) >= 1
    P = [
        2.46196981473530512524E-10,
        5.64189564831068821977E-1,
        7.46321056442269912687E0,
        4.86371970985681366614E1,
        1.96520832956077098242E2,
        5.26445194995477358631E2,
        9.34528527171957607540E2,
        1.02755188689515710272E3,
        5.57535335369399327526E2,
    ]

    Q = [
        1.32281951154744992508E1,
        8.67072140885989742329E1,
        3.54937778887819891062E2,
        9.75708501743205489753E2,
        1.82390916687909736289E3,
        2.24633760818710981792E3,
        1.65666309194161350182E3,
        5.57535340817727675546E2,
    ]

    # approximation for abs(a) >= 8
    R = [
        5.64189583547755073984E-1,
        1.27536670759978104416E0,
        5.01905042251180477414E0,
        6.16021097993053585195E0,
        7.40974269950448939160E0,
        2.97886665372100240670E0,
    ]

    S = [
        2.26052863220117276590E0,
        9.39603524938001434673E0,
        1.20489539808096656605E1,
        1.70814450747565897222E1,
        9.60896809063285878198E0,
        3.36907645100081516050E0,
    ]

    # Shortcut special cases
    if a == 0:
        return 1
    if a >= MAXVAL:
        return 0
    if a <= -MAXVAL:
        return 2

    x = a
    if a < 0:
        x = -a

    # computationally cheaper to calculate erf for small values, I guess.
    if x < 1:
        return 1 - erf(a)

    z = -a * a

    z = math.exp(z)

    if x < 8:
        p = _polevl(x, P, 8)
        q = _p1evl(x, Q, 8)
    else:
        p = _polevl(x, R, 5)
        q = _p1evl(x, S, 6)

    y = (z * p) / q

    if a < 0:
        y = 2 - y

    return y
Port of cephes ``ndtr.c`` ``erfc`` function.

    See https://github.com/jeremybarnes/cephes/blob/master/cprob/ndtr.c