recursively constructs A line for g; i = len(g)-1

def A(g,i):
    """recursively constructs A line for g; i = len(g)-1"""
    g1 = g&(2**i)
    if i:
        n = Awidth(i)
        An = A(g,i-1)
        if g1:
            return An<<n | An
        else:
            return int('1'*n,2)<<n | An
    else:
        if g1:
            return int('00',2)
        else:
            return int('10',2)