Morphologically conditioned allomorphy may involve suppletion (as in "go-Ø/wen-t") or readjustment rules that apply in the context of certain Vocabulary items (as in "buy-Ø/bough-t"). Suppletion and readjustment rules apply to a terminal node and its associated Vocabulary item – unlike affixation, which combines this terminal node with a separate terminal node that has its own distinct (though potentially null) Vocabulary item. Suppletion arises from the competition of Vocabulary items for insertion into a terminal node. Competition involving root Vocabulary items is a topic of ongoing research, however. Early work in Distributed Morphology suggests that a single, abstract lexical root appears in the syntax; in this view, roots do not compete for insertion into root nodes, but exist in free variation, constrained only by semantic and pragmatic well-formedness. Subsequent research has suggested that the distribution of root Vocabulary items can be grammatically restricted (Embick 2000, Pfau 2000, Marantz 2013); this means that roots may be featurally restricted and thus subject to competition. The issue of whether root alternations such as "buy-Ø/bough-t" are better handled by suppletion or readjustment rules remains a topic of debate (Embick & Marantz 2008, Siddiqi 2009, Bonet & Harbour 2012).