  Pumping routes of masers can in principle be recovered from a small matrix of
master equations, at an advanced stage of elimination, by tracing back the
coefficients to a set of unmodified all-process rate coefficients, drawn from
those which appeared in the original set of master equations, prior to any
elimination operations. The traceback is achieved by logging the operations
carried out on each coefficient. There is no guarantee that a pumping scheme
can be represented as a small set of important routes in this way. In the
present work, the traceback method is applied to a model which is typical of a
large volume of parameter space which produces very strong inversions in the
main lines of the rotational ground state of OH, at 1665- and 1667-MHz. For
both lines, the pumping scheme is largely restricted to the $^{2}\Pi_{3/2}$
stack of rotational levels, and it is possible to list a comparatively small
set of routes (less than ten) which provide more than 80 per cent of the
inversion. In both cases, the strongest, and simplest, route consists of a
radiative upward stage, to the $^{2}\Pi_{3/2}, J=5/2$ rotational level,
followed by a collisional de-excitation to the rotational ground state.
