Implicit ANDs between shape expressions

[labra]

Currently, ShExC allows to simplify shape expressions when the first one is a node constraint omitting the keyword AND.

So the following two shapes are equivalent:

<S1> IRI AND { :q xsd:string }
<S2> IRI { :q xsd:string }

However, if the left part of the AND is another shape, then it is not possible to omit the AND:

<S1> { :p xsd:string } AND { :q xsd:string }
<S2> { :p xsd:string } { :q xsd:string }  # Syntax error

Although I think we can leave as is for version 2.0, we should consider if it is better to be extend the simplification and to enable implicit ANDs between shape expressions in general.

However, I am not sure if allowing it would increase readability, so I open this issue to open a discussion about it for future versions.