  For a manifold $M$ with an integral closed 3-form $\omega$, we construct a
$PU(H)$-bundle and a Lie groupoid over its total space, together with a curving
in the sense of gerbes. If the form is non-degenerate, we furthermore give a
natural Lie 2-algebra quasi-isomorphism from the observables of $(M,\omega)$ to
the weak symmetries of the above geometric structure, generalising the
prequantisation map of Kostant and Souriau.
