def de_bruijn(k, n):
"""
de Bruijn sequence for alphabet k
and subsequences of length n.
"""

alphabet = k
k = len(k)

a = [0] * k * n
sequence = []

def db(t, p):
    if t > n:
        if n % p == 0:
            sequence.extend(a[1:p + 1])
    else:
        a[t] = a[t - p]
        db(t + 1, p)
        for j in range(a[t - p] + 1, k):
            a[t] = j
            db(t + 1, t)
db(1, 1)
print(len(sequence))
#return ", ".join(alphabet[i] for i in sequence)
return "".join(alphabet[i] for i in sequence)

print(de_bruijn("01", 2))