Make "*-on-and-only" an AttributeDescription option

[Anne Anderson <Anne.Anderson@Sun.COM>]

I would like to propose that we add an optional "OneAndOnly" XML
attribute to the <AttributeDescriptor> element.  If this
attribute is "true", it causes the <AttributeDescriptor> to
evaluate to a single element of its DataType, rather than a bag
of such elements.  If this XML attribute is used, and the
<AttributeDescriptor> matches no element or more than one
element, then the evaluation result will be "Indeterminate".  The
default value of the OneAndOnly Attribute is "false".

The intent is that

  <SomeAttributeDescriptor AttributeId="x" DataType="y" OneAndOnly="true"/>

be exactly equivalent to

  <Apply FunctionId="y-one-and-only">
     <SomeAttributeDescriptor AttributeId="x" DataType="y""/>
  </Apply>

Rationale:

  All our standard functions except for the specific bag
  functions expect a single value, so the case for expecting and
  wanting a single attribute is extremely common.  It is awkward,
  verbose, and probably confusing to wrap all instances of
  <AttributeDescriptor> with an <Apply> of a "*-one-and-only"
  function.

Anne
-- 
Anne H. Anderson             Email: Anne.Anderson@Sun.COM
Sun Microsystems Laboratories
1 Network Drive,UBUR02-311     Tel: 781/442-0928
Burlington, MA 01803-0902 USA  Fax: 781/442-1692