The singleton degrees of the Sigma-0-2 sets are not dense

Answering an open question raised by Cooper, we show that there exist Δ-0-2 sets D and E such that the singleton degree of E is a minimal cover of the singleton degree of D. This shows that the Σ0-2 singleton degrees, and the Δ0-2 singleton degrees, are not dense (and consequently the Π-0-2 Q-degrees, and the Δ-0-2 Q-degrees, are not dense). Moreover, D and E can be built to lie in the same enumeration degree.}