Add an index to the filter, returning an integer that can be used to identify the index in the selection functions.

public long AddIndex(ref String indexPath);

Public Function AddIndex(ByRef indexPath As String) As long

Integer to be used as whichIndex in the Select methods. 

If the index is already present in the search filter, the index is not added a second time, and the identifier for the existing instance of the index is returned. Therefore, addIndex() can be used to get the identifier for an index that has previously been added to the SearchFilter.