What is "cumomer"

A cumomer is a cumulative isotopomer and is a concept that relates to metabolic flux analysis. Specifically if we consider a molecule as a set of atoms, any of which could be ( isotopically) labeled, the cumomers are a set of isotopomers with particular positions of 13C-labels grouped into different levels depending on the amount of labeled atoms. At level 0, any position can be either 12C or 13C, at level 1 one position is 13C whether the others may or may not be labeled and so on.