[cctbxbb] getting indices of selected grid elements of a map

Hi, I am wondering if there is an efficient way to retrieve the 3D indices of a selection from a map. In the example on https://cci.lbl.gov/docs/cctbx/doc_low_flex_advanced/#getting-indices-of-sel... it is shown how to list the indices of a subset of grid elements of the map as in print(list(isel)) But isel is a 1D representation of 3d map indices. What if I want to get hold of the list of 3D indices corresponding to the indices in the isel array? Is there a fast boost-python way of doing this or would I have to do a 3D loop over the unit cell grid numbers to see if enumerated grid elements matches any of the elements in the isel array? Many thanks, Rob