[cctbxbb] Extracting data from MTZ

Hi all, I have a script that extracts Fobs and Fcalc from an MTZ file produced by Refmac for some analysis. I was just doing this in a trivial way: fobs = m.extract_reals("F").data fc = m.extract_reals("FC_ALL").data and this worked fine, until I came across a file with missing observations, so that the lengths of fobs and fc do not match. Can anybody advise a better way to do this please? I'm inexperienced with iotbx.mtz. I don't need anything other than the values of the structure factors, but I need each element of fobs and fc to contain the equivalent values. Thanks -- David