We have a set of n sequential processes $p_{1},...,p_n$ , that access $m$ concurrent objects $X_1,...,X_m$ by invoking operations on the form `Xi.op(args)(ret)`.

When invoked by $p_j$, the invocation `Xi.op(args)(ret)` is modeled by two events: `inv[Xi.op(args) by pj]` and `res[Xi.op(ret) to pj]`.

A **history** (or **trace**) is a pair $\hat{H}=(H, <_{H})$ where H is a set of events and $<_{H}$ is a total order on them.