Preponement
We refine an analysis of the main result of
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H. Doornbos and B. von Karger
On the Union of Well-Founded Relations
Logic Journal of the IGPL,
volume 6(2), pp. 195-201, 1998
(doi:10.1093/jigpal/6.2.195)
showing how two simple transformation steps allow one
to transform any reduction sequence comprising
two kinds of reduction steps, into one in which all
reduction steps of the first kind precede those of the
second kind. Several known results are shown to be corollaries.