def split(a):
    """
    Split float64 in two aligned parts.

    Uses the procedure of Shewchuk, 1997,
    Discrete & Computational Geometry 18(3):305-363
    http://www.cs.berkeley.edu/~jrs/papers/robustr.pdf

    """
    c = 134217729. * a  # 2**27+1.
    abig = c - a
    ah = c - abig
    al = a - ah
    return ah, al