NXDL: The `<symbols />` block must appear before the `<doc />` block.

[prjemian]

The <symbols /> block must appear before the <doc /> block.

Can this rule be relaxed?

definitions/nxdl.xsd

Lines 200 to 219 in ffa3198

	<xs:sequence>
	<xs:element name="symbols" type="nx:symbolsType" minOccurs="0" maxOccurs="1">
	<xs:annotation>
	<xs:documentation>
	Use a ``symbols`` list
	to define each of the mnemonics that
	represent the length of each dimension in a vector or array.
	</xs:documentation>
	</xs:annotation>
	</xs:element>
	<xs:group ref="nx:groupGroup" minOccurs="0" maxOccurs="unbounded">
	<xs:annotation>
	<xs:documentation>
	In addition to an optional ``symbols`` list,
	a ``definition`` may contain any of the items
	allowed in a ``group``.
	</xs:documentation>
	</xs:annotation>
	</xs:group>
	</xs:sequence>

Originally posted by @prjemian in #996 (comment)

[prjemian]

The xs:sequence element is a child of xs:complexType. Check the documentation for alternatives:

type	children	documentation
xs:complexType	(annotation?,(simpleContent|complexContent|((group|all|choice|sequence)?,((attribute|attributeGroup)*,anyAttribute?))))	The complexType element defines a complex type. A complex type element is an XML element that contains other elements and/or attributes.
xs:all	(annotation?,element*)	The all element specifies that the child elements can appear in any order and that each child element can occur zero or one time.
xs:sequence	(annotation?,(element|group|choice|sequence|any)*)	The sequence element specifies that the child elements must appear in a sequence. Each child element can occur from 0 to any number of times.

[benajamin]

xs:all appears to fit the situation.

[prjemian]

The complication is that doc is defined in a separate structure:

definitions/nxdl.xsd

Lines 491 to 499 in ffa3198

	<xs:group name="groupGroup">
	<xs:annotation>
	<xs:documentation>
	A ``groupGroup`` defines the allowed children of a
	``group`` specification.
	</xs:documentation>
	</xs:annotation>
	<xs:sequence>
	<xs:element name="doc" type="nx:docType" minOccurs="0" maxOccurs="1">

[prjemian]

xs:all appears to fit the situation.

Not really, since xs:all only allows children of element type.

[prjemian]

Choices (for children of complexType) are simpleContent|complexContent|(group|all|choice|sequence)?

[prjemian]

Maybe, we could extend groupGroup for its use in definitionType and move symbols into that extension, allowing it to appear in any order as defined in the extension.

[prjemian]

FYI: xs:all could replace xs:sequence (lines 1138 & 1176) in the symbolsType structure:

definitions/nxdl.xsd

Line 1138 in ffa3198

<xs:sequence>

definitions/nxdl.xsd

Line 1176 in ffa3198

</xs:sequence>

so that doc and symbol elements could appear in either order.

[prjemian]

Not easy to do and keep the groupGroup structure. Part of the problem is that xs:all only has element content and each of those elements has maxOccurs="1" as an unchangeable limitation.

Setting this aside for another time.