  Message Sequence Charts & Sequence Diagrams are graphical models that
represent the behavior of distributed and concurrent systems via the scheduling
of discrete and local emission and reception events. We propose an Interaction
Language (IL) to formalize such models, defined as a term algebra which
includes strict and weak sequencing, alternative and parallel composition and
four kinds of loops. This IL is equipped with a denotational-style semantics
associating a set of traces (sequences of observed events) to each interaction.
We then define a structural operational semantics in the style of process
algebras and formally prove the equivalence of both semantics.
