Some questions on ShEx specification

I have a small set of questions about the ShEx abstract syntax
specification:

1.- Is it necessary the field "id: Label" in the arcrule? (
http://www.w3.org/2013/ShEx/Definition#as_defn_ArcRule)

2.- Should we have Set[RDFNode] in ValueSet instead of Set[IRI] so we could
define, for example:
    <a> { :p (1 2) }  ? (
http://www.w3.org/2013/ShEx/Definition#as_defn_ArcRule)

3.- I think the definition of Schema could be replaced by Set[Shape] ?

4..- I also think, "start" should be added to the Schema abstract
syntax...such as Schema = (shapes: Set[Shape], start = Label) ?

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Best regards, Jose Labra