  Computation can be considered by taking into account two dimensions:
extensional versus intensional, and sequential versus concurrent. Traditionally
sequential extensional computation can be captured by the lambda-calculus.
However, recent work shows that there are more expressive intensional calculi
such as SF-calculus. Traditionally process calculi capture computation by
encoding the lambda-calculus, such as in the pi-calculus. Following this
increased expressiveness via intensionality, other recent work has shown that
concurrent pattern calculus is more expressive than pi-calculus. This paper
formalises the relative expressiveness of all four of these calculi by placing
them on a square whose edges are irreversible encodings. This square is
representative of a more general result: that expressiveness increases with
both intensionality and concurrency.
