Returns all Principal objects assigned a given Permission in the access
control list. The Principals returned are those that have been granted
either the supplied permission, or a permission implied by the supplied
permission. Principals are not "expanded" if they are a role or group.
Adds an ACL entry to this ACL. An entry associates a principal (e.g., an
individual or a group) with a set of permissions. Each principal can have
at most one positive ACL entry, specifying permissions to be granted to
the principal. If there is already an ACL entry already in the ACL, false