def subsequence(str1):
    if (len(str1)==0):
        return [" "]
    small = subsequence(str1[1:len(str1)])
    result = [" "]* (2* len(small))
    k=0
    for i in range(len(small)):
        result[k]=small[i]
        k=k+1
    for i in range(len(small)):
        result[k]=str1[0]+small[i]
        k=k+1
    return result

print(subsequence("net"))