which pi formula is given in the decimal module documentation?

[Anh Hai Trinh]

I'm just curious which formula for pi is given here: <http://
docs.python.org/library/decimal.html#recipes>?

def pi():
    """Compute Pi to the current precision.

    >>> print pi()
    3.141592653589793238462643383

    """
    getcontext().prec += 2  # extra digits for intermediate steps
    three = Decimal(3)      # substitute "three=3.0" for regular
floats
    lasts, t, s, n, na, d, da = 0, three, 3, 1, 0, 0, 24
    while s != lasts:
        lasts = s
        n, na = n+na, na+8
        d, da = d+da, da+32
        t = (t * n) / d
        s += t
    getcontext().prec -= 2
    return +s               # unary plus applies the new precision


It looks like an infinite series with term `t`, where`n` = (2k-1)^2
and `d` = d = 4k(4k+2) for k = 1... Does it have a name?