js/leaflet.markercluster.js
function (previousBounds, zoomLevel, exceptBounds) {
    var m, i;
    this._recursively(previousBounds, -1, zoomLevel - 1,
      function (c) {
        //Remove markers at every level
        for (i = c._markers.length - 1; i >= 0; i--) {
          m = c._markers[i];
          if (!exceptBounds || !exceptBounds.contains(m._latlng)) {
            c._group._featureGroup.removeLayer(m);
            if (m.setOpacity) {
              m.setOpacity(1);
            }
          }
        }
      },
      function (c) {
        //Remove child clusters at just the bottom level
        for (i = c._childClusters.length - 1; i >= 0; i--) {
          m = c._childClusters[i];
          if (!exceptBounds || !exceptBounds.contains(m._latlng)) {
            c._group._featureGroup.removeLayer(m);
            if (m.setOpacity) {
              m.setOpacity(1);
            }
          }
        }
      }
    );
  }
function (previousBounds, zoomLevel, exceptBounds) {
    var m, i;
    this._recursively(previousBounds, -1, zoomLevel - 1,
      function (c) {
        //Remove markers at every level
        for (i = c._markers.length - 1; i >= 0; i--) {
          m = c._markers[i];
          if (!exceptBounds || !exceptBounds.contains(m._latlng)) {
            c._group._featureGroup.removeLayer(m);
            if (m.setOpacity) {
              m.setOpacity(1);
            }
          }
        }
      },
      function (c) {
        //Remove child clusters at just the bottom level
        for (i = c._childClusters.length - 1; i >= 0; i--) {
          m = c._childClusters[i];
          if (!exceptBounds || !exceptBounds.contains(m._latlng)) {
            c._group._featureGroup.removeLayer(m);
            if (m.setOpacity) {
              m.setOpacity(1);
            }
          }
        }
      }
    );
  }
exceptBounds: If set, don't remove any markers/clusters in it