Simple grouping with untyped properties or open types

[HeikoTheissen]

In the simple grouping algorithm, the last sub-bullet point of the projection function says:

Otherwise, if $p'$ is a prefix of a path in $G$, let $v[q]=s_G (u[q],p')$

and the recursively called function $s_G$ assumes that its first argument is a structured instance. But if $q$ is an untyped property or a property of an open type, $u[q]$ can also be primitive¹, and then the first bullet point does not make sense:

Let $v$ be an instance of the type of $u$ without properties and without entity-id.

I suggest to rephrase the last sub-bullet point so that the recursive call is omitted in such cases. This is the least invasive change and its leads to $q$ being omitted from the projection.

¹ Of course $u[q]$ can also be primitive if $q$ declares a primitive type, but the problem occurs only if $p'$ is a prefix of a path in $G$, which means for a grouping criterion of the form $q/r$ or $s/q/r$. And if $q$ declares a primitive type, $q/r$ does not make sense at all.