  The main aim of the paper is to develop the "Floer theory" associated to
Calabi-Yau 3-folds, exending the analogy of Thomas' "holomorphic Casson
invariant". The treatment in the body of the paper is largely formal, assuming
appropriate compactness properties of moduli spaces of $G_{2}$-instantons, but
in the last section we make some remarks about these compactness isssues.
Section 3 of the paper contains a general dscussion of deformations of the
equations, for gauge field and submanifolds, associated to manifolds with
exceptional holonomy.
