public void add(Permission permission) {
        Set<Permission> permissions = getPermissions();
        if (permissions == null) {
            permissions = new LinkedHashSet<Permission>();
            setPermissions(permissions);
        }
        permissions.add(permission);
    }
A simple representation of a security role that has a name and a collection of permissions.  This object can be
used internally by Realms to maintain authorization state.

@since 0.2
core/src/main/java/org/apache/shiro/authz/SimpleRole.java