(By Hedvig Skirgård): "Is there a difference between the marking of NP coordination (John and Mary went to market.) and the marking of comitative phrases (John with Mary went to market.)? In some languages the only way to coordinate noun phrases is with a comitative phrase, i.e. the only way to say ‘X and Y’ is ‘X with Y’. To see whether an element is comitative or coordinating you can check case marking and agreement. For example, if X is the element in question and in the dative you get ‘John-DAT X Mary-DAT’, then X is likely coordinating. If you get ‘John-DAT X Mary’, X is likely comitative. If there is agreement, if you get ‘John X Mary verb-PL’ rather than 'John X Mary verb-SG' (just like ‘John X suitcase verb-SG’), this suggests that X is coordinating."