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Given a reference set 
,
we indicate the elements of
which belong to a certain class of
and those which do not belong to it.
This class is then a subset of
with the usual meaning in set theory, and will be qualified as classical
or crisp in the following paragraphs.
If the membership of some elements of
to a class is not absolute, we can indicate to which degree each element
belongs to this class. This class is then a fuzzy
subset of .
