  We propose and study the notions of behaviour type and composition operator
making a first step towards the definition of a formal framework for studying
behaviour composition in a setting sufficiently general to provide insight into
how the component-based systems should be modelled and compared. We illustrate
the proposed notions on classical examples (Traces, Labelled Transition Systems
and Coalgebras). Finally, the definition of memoryless glue operators, takes us
one step closer to a formal understanding of the separation of concerns
principle stipulating that computational aspects of a system should be
localised within its atomic components, whereas coordination layer responsible
for managing concurrency should be realised by memoryless glue operators.
