  In this note we present a new construction of the string group that ends
optionally in two different contexts: strict diffeological 2-groups or
finite-dimensional Lie 2-groups. It is canonical in the sense that no choices
are involved; all the data is written down and can be looked up (at least
somewhere). The basis of our construction is the basic gerbe of Gawedzki-Reis
and Meinrenken. The main new insight is that under a transgression-regression
procedure, the basic gerbe picks up a multiplicative structure coming from the
Mickelsson product over the loop group. The conclusion of the construction is a
relation between multiplicative gerbes and 2-group extensions for which we use
recent work of Schommer-Pries.
