Razborov closure

Fix integers $r$ and $l$. The Razborov closure of a family of vertex sets of size at most $l$ repeatedly adjoins a set $W$ whenever there are $r$ existing sets $W_1,\dots ,W_r$ whose pairwise intersections are contained in $W$. A family equal to its closure is called $r$-closed.

An $r$-closed family has at most $(r-1{)}^k$ inclusion-minimal members of cardinality $k$. This limits the number of cliques accepted by a proper closed approximation.