<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def serialize_dict_keys(d, prefix=""):
 """returns all the keys in a dictionary. ['a', 'a.b', 'a.b.c', 'a.b.b'] """

    keys = []
    for k, v in d.iteritems():
        fqk = '%s%s' % (prefix, k)
        keys.append(fqk)
        if isinstance(v, dict):
            keys.extend(serialize_dict_keys(v, prefix="%s." % fqk))

    return keys