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Element: optional

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Attributes: datatypeLibrary | ns |
Children: <other> | <pattern> ( element, attribute*, group*, interleave*, choice, optional, zeroOrMore, oneOrMore*, list*, mixed, ref*, parentRef, empty*, text*, value*, data*, notAllowed, externalRef, grammar ) |
Parents: attribute | choice | define | element | except | group | interleave | list | mixed | oneOrMore | optional | start | zeroOrMore |

<element name="optional">
     <ref name="common-atts"/>
     <ref name="open-patterns"/>
</element>

Occurs in Schema Tutorial examples:
Exact number of elements from any namespace
Uniqueness
Uniqueness - comparison of numbers