Description: Define the set of closed subspaces of a Hilbert space. A closed subspace
is one in which the limit of every convergent sequence in the subspace
belongs to the subspace. For its membership relation, see isch . From
Definition of Beran p. 107. Alternate definitions are given by isch2 and isch3 . (Contributed by NM, 17-Aug-1999)(New usage is discouraged.)