tail recursion

[ldemailly]

would be neat to get

// Γ(x + 1/2) ~ Γ(x)x^(1/2) = (x-1)!√x
// Γ(x + 1/2) = (2x - 1)!! * 2^-x * √π
f=func(i,n, prod) {
    //log(i, prod)
	if (i==n+1) {
        return 1./(prod*prod*n)
    }
    f(i+1, n, prod * ( 1 - 1./(2*i)))
}
n=200000
f(1,n,1)

to work for larger Ns withou

runtime: goroutine stack exceeds 1000000000-byte limit
runtime: sp=0x14020408380 stack=[0x14020408000, 0x14040408000]
fatal error: stack overflow