def angle(v):
    """
    Return the argument of the vector ``v``.
    """
    from math import acos, asin, sqrt, pi
    x = float(v[0])
    y = float(v[1])
    r = sqrt(x ** 2 + y ** 2)
    if abs(x) >= abs(y):
        if x >= 0:
            return asin(y / r) / pi
        else:
            return -asin(y / r) / pi
    elif y >= 0:
        return acos(x / r) / pi
    else:
        return -acos(x / r) / pi
