  Collisions between complex nuclei may give rise to their total or partial
fusion. The latter case is found experimentally to gain importance when one of
the colliding nuclei is weakly bound. It has been commonly assumed that the
partial fusion mechanism is a two-step process, whose first step is the
dissociation of the weakly bound nucleus, followed by the capture of one of the
fragments. To assess this interpretation, we present the first implementation
of the three-body model of inclusive breakup proposed in the 1980s by Austern
et al. [Phys. Rep. 154, 125 (1987)] that accounts for both the direct,
one-step, partial fusion and the two-step mechanism proceeding via the
projectile continuum states. Contrary to the widely assumed picture, we find
that, at least for the investigated cases, the partial fusion is largely
dominated by the direct capture from the projectile ground-state.
