Joint Participation As Accompaniment

There	have been	more clashes	between Iraqi and British forces
	Process: existential	Existent	Accompaniment: comitative

This clause is agnate with:

Iraqi and British forces	have clashed
Actor	Process: material

Iraqi forces	have clashed	with British forces
Actor	Process: material	Accompaniment

British forces	have clashed	with Iraqi forces
Actor	Process: material	Accompaniment

Halliday & Matthiessen (1999: 174):

The circumstance of Accompaniment does not correspond to any one particular participant rôle. Rather, it corresponds to an extending of the participant itself, by addition or variation… Grammatically, the analogous type of participant is one represented by a nominal group complex.

Halliday & Matthiessen (2014: 324):

Accompaniment is a form of joint participation in the process and represents the meanings ‘and’, ‘or’, ‘not’ as circumstantials;

See also Degree of Involvement.