<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def normalize_missing(xs):
 """Normalize missing values to avoid string 'None' inputs. """

    if isinstance(xs, dict):
        for k, v in xs.items():
            xs[k] = normalize_missing(v)
    elif isinstance(xs, (list, tuple)):
        xs = [normalize_missing(x) for x in xs]
    elif isinstance(xs, six.string_types):
        if xs.lower() in ["none", "null"]:
            xs = None
        elif xs.lower() == "true":
            xs = True
        elif xs.lower() == "false":
            xs = False
    return xs