Introduction

This is a schema for representing infosets explicitly in XML. There are two main versions: one for the basic infoset, and one for the post-schema-validation (PSV) infoset.

One reason for producing this schema was to allow comparison of the infosets generated by different processors (including parsers and schema validators). It has also proved useful for finding flaws in the infoset and schema specifications themselves.

Notes

All structures are represented as elements. There are two reasons for this:

• It avoids the need to decide each case individually.
• It allows all structures to be nulled with xsi:null.

A type is declared for each info item and property. Type names are camel-case with an initial capital. Element names are camel-case with an initial lower-case letter.

All properties are represented as elements whose name is the property name. These elements are globally declared (so properties with the same name must have the same type; this is true for the basic infoset and we can make it true for the PSV infoset). Their types fall into several categories:

• Atomic (strings, enumerations and booleans):
  The element has a simple type.
• (Ordered) lists of info items:
  The element is a container for a sequence of elements which represent the info items.
• (Unordered) sets of info items:
  As for lists. The items are sorted into a canonical order. For attributes and namespaces this is the same as the order in Canonical XML. For other cases we will specify an order.
• Single info items:
  Surprisingly, there are none of these in the basic infoset, except in cases where a pointer is used (see below). They do occur in the PSV infoset. If the property must be of a fixed type, and the name of the property is the same as the name of the type, the element is the info item itself (the PSV [annotation] property is an example). If the property can be any of several different info item types (including the case where it may be either a real info item or a pointer to one), the element contains the info item (the PSV [type definition] property is an example).
• References to info items:
  Where the very same info item appears in two or more places, we specify that one contains the real value and the others contain pointers. All info items that are pointed to have attributes named id. A property pointing to an info item contains an element named pointer which has an attribute named ref corresponding to the pointed-to item. Identity constraints in the schema enforce the correspondence (not done for schema components yet).
  When there is a natural home for the real definitions it is used. In particular, entity and notation definitions reside in the [entities] and [notations] properties of the document info item. Global schema components reside in the [schema components] property of the schema information info item, others reside in the component in which they are defined (for example a local element declaration will reside in a particle).
  Here is a table of standard info items that are sometimes referred to by pointers:
  

  Info item	Home item[property]	Pointing item[property]
  Entity	document[entities]	skipped entity[entity] DTD[external DTD] Entity Start/End[entity]
  Notation	document[notations]	entity declaration[notation]

  
  Here is a table of PSV info items:
  

  Info item	Home item[property]	Pointing item[property]
  Attribute Declaration	schema information[schema components] attribute group[attribute use pairs] complex type[attribute use pairs]	attribute group[attribute use pairs] complex type[attribute use pairs]
  Element Declaration	schema information[schema components] particle[term]	particle[term]
  Simple Type Definition	schema information[schema components] attribute declaration[type definition] element declaration[type definition] complex type definition[base type definition] simple type definition[base type definition] simple type definition[primitive type definition] simple type definition[item type definition] simple type definition[union type definition]	attribute declaration[type definition] element declaration[type definition] simple type definition[base type definition] simple type definition[primitive type definition] simple type definition[item type definition] simple type definition[union type definition]
  Complex Type Definition	schema information[schema components] element declaration[type definition]	element declaration[type definition] complex complex type definition[base type definition]
  Attribute Group Definition	schema information[schema components]	(none)
  Model Group Definition	schema information[schema components]	(none)
  Model Group	model group definition[model group]	particle[term]
  Particle	model group[particles] complex type definition[content type]	(none)
  Wildcard	complex type definition[attribute wildcard] attribute group definition[attribute wildcard] particle[term]	(none)
  Notation	schema information[schema components]	?

  
  
• Odd cases:
  The PSV infoset has some odd cases, such as "either global or a complex type definition". We will have to deal with them case-by-case.

Both the basic and PSV infosets have an item called Notation Declaration. Something will have to be done about this in the Schema spec. For now I call the schema version S-NotationDeclaration.

(Say something about subsets and supersets.)