 and $\lim_{h \to 0} \sqrt{0+h} =  \lim_{h\to 0}(0 + h/0)$. For ease of computation, we replace $\phi'$ by $\psi$, to be restored at the last step. $$\label{mucom}     \lim_{a \to b} \frac{\sqrt{{\ensuremath{D_\phi}}(a,q)} - \sqrt{{\ensuremath{D_\phi}}(b,q)}}{\sqrt{{\ensuremath{D_\phi}}(b,a)}}      