protected void buildIndexMap()
	{
		//iterate over the names of fields with indices
		Set<String> keys = indices.keySet();
		for(String field:keys)
		{
			List<String>indiceList = indices.get(field);
			if(indiceList!=null)
			{
				for(String ind:indiceList)
				{
					//add field to the list of fields indexed by index ind
					List<String>fieldNames = indexMap.get(ind);
					if(fieldNames==null)
					{
						fieldNames = new ArrayList<String>();
					}
					fieldNames.add(field);
					indexMap.put(ind, fieldNames);
				}
			}
		}
	}