Proper subset
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A
set
X
is a
proper subset
, or strict subset, of a set
Y
if
X
is a
subset
of
Y
and
there is an element in
Y
that is not in
X
Examples:
{1, 2} is a proper subset of {1, 2, 3}
{1, 2} is
not
a proper subset of {1, 2}