DictsOut is a copy of DictsIn, where each dict
contains all keys appearing in all dicts of DictsIn. Values
for keys that are added to a dict are produced by calling OnEmpty
as below. The predicate dict_fill/4
provides an implementation that fills all new cells with a predefined
value.
dicts_to_same_keys(+DictsIn,
:OnEmpty, -DictsOut)