How to use the ssb-friends/block.isWanted function in ssb-friends

FlatMap(function (v) {
          if(!v) return []

          //this code handles real time streaming of the hops map.
          function push (to, hops) {
            out.push(meta ? {id: to, hops: hops} : to)
          }

          var out = [], g = post.value

          //the edge has already been added to g
          if(!reachable) {
            reachable = F.reachable(g, start, block)
            for(var k in reachable)
              if(block.isWanted(reachable[k]))
                push(k, reachable[k][0])
          } else {
            var _reachable = F.reachable(g, start, block)
            var patch = F.diff(reachable, _reachable, block)
            for(var k in patch) {
              if(patch[k] == null || patch[k][0] == null || patch[k][0] > patch[k][1])
                push(k, -1)
              else if(block.isWanted(patch[k]))
                push(k, patch[k][0])
            }
            reachable = _reachable
          }
          return out
        })