<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _erf(x):
 """ Port of cephes ``ndtr.c`` ``erf`` function. See https://github.com/jeremybarnes/cephes/blob/master/cprob/ndtr.c """

    T = [
        9.60497373987051638749E0,
        9.00260197203842689217E1,
        2.23200534594684319226E3,
        7.00332514112805075473E3,
        5.55923013010394962768E4,
    ]

    U = [
        3.35617141647503099647E1,
        5.21357949780152679795E2,
        4.59432382970980127987E3,
        2.26290000613890934246E4,
        4.92673942608635921086E4,
    ]

    # Shorcut special cases
    if x == 0:
        return 0
    if x >= MAXVAL:
        return 1
    if x <= -MAXVAL:
        return -1

    if abs(x) > 1:
        return 1 - erfc(x)

    z = x * x
    return x * _polevl(z, T, 4) / _p1evl(z, U, 5)