  The pi-calculus is a widely used process calculus, which models
communications between processes and allows the passing of communication links.
Various operational semantics of the pi-calculus have been proposed, which can
be classified according to whether transitions are unlabelled (so-called
reductions) or labelled. With labelled transitions, we can distinguish early
and late semantics. The early version allows a process to receive names it
already knows from the environment, while the late semantics and reduction
semantics do not. All existing reversible versions of the pi-calculus use
reduction or late semantics, despite the early semantics of the (forward-only)
pi-calculus being more widely used than the late. We define piIH, the first
reversible early pi-calculus, and give it a denotational semantics in terms of
reversible bundle event structures. The new calculus is a reversible form of
the internal pi-calculus, which is a subset of the pi-calculus where every link
sent by an output is private, yielding greater symmetry between inputs and
outputs.
