Halliday & Matthiessen (1999: 168):
In some special cases, the generalisation of the Goal across a class of entities is shown by treating it as a mass, dispensing with the plural marker;They often shoot duck during the winter months.Such a Goal may even be incorporated into the Process, as in he is baby-sitting (and even who’s baby-sitting me this evening?); this is a restricted option with figures realised as ranking clauses, but not uncommon where the figure is used to qualify an element and is realised by a pre-modifying clause, e.g. a fun-loving colleague, a wood-burning stove.