Geometry of Interaction, combined with translation of lambda-calculus into MELL proof nets, has enabled an unconventional approach to program semantics. Danos and Regnier, and Mackie pioneered the approach, and introduced the so-called token-passing machines.
It turned out that the unconventional token-passing machines can be turned into a graphical realisation of conventional reduction semantics, in a simple way. The resulting semantics can be more convenient than the standard (syntactical) reduction semantics, in analysing local behaviour of programs. I will explain how, in particular, the resulting graphical reduction semantics can be used to reason about observational equivalence between programs.[-]