constructs the BB context

def BB(n):
    """constructs the BB context"""
    if (n<=1):return Context('0\n1')
    else:
        BB1=BB(n-1)
        AA1=AA(n-1)
        r1 = C1((n-1)*2**(n-2),2**(n-1)) - AA1 - BB1
        r2 = BB1 - C1(2**(n-1),2**(n-1)) - BB1;
        return r1 + r2