Unimplemented mapping API:
a.copy()    a (shallow) copy of a   
a.update([b])   updates a with key/value pairs from b, overwriting existing keys, returns None
a.fromkeys(seq[, value])    Creates a new dictionary with keys from seq and values set to value
a.setdefault(k[, x])    a[k] if k in a, else x (also setting it)
a.pop(k[, x])   a[k] if k in a, else x (and remove k)
a.popitem()     remove and return an arbitrary (key, value) pair
a.iteritems()   return an iterator over (key, value) pairs
a.iterkeys()    return an iterator over the mapping's keys
a.itervalues()  return an iterator over the mapping's values
