closure(A,L) or closure(A,I)
The closure of a set of indices $L$ consists of (indices of) all hyperplanes that contain the intersection of the given ones.
Equivalently, the closure of $L$ consists of all hyperplanes whose defining linear forms are in the span of the linear forms indexed by $L$.





The closure of a linear ideal $I$ is the flat consisting of all the hyperplanes in $\mathcal A$ whose defining forms are also in $I$.