The mechanism named ``Superimposed coding'' [11] allows to
store a set of data of variable size into a bit-string of fixed
size. It is then possible, with a simple test, to estimate the
probability that an element is in the set of data (which depends
on the size of the result bit-string and on the number of data).
This probability is equal to 0 if the output of the test is
``no''.

More precisely, the result is an -bit string named . We note
where each
.
Initially, is set to . We have then elements
of various size and we note the set of data
. Moreover, let us define hash
functions
where each
with .

For we compute
and for
every we put to 1 the bit where
.

To know if the element is in the set of data
, we compute for every
and if there is an element
such as
then
. If not, then with an error
probability of about
.