
cts:value-co-occurrences( $range-index-1 as cts:reference, $range-index-2 as cts:reference, [$options as xs:string*], [$query as cts:query?], [$quality-weight as xs:double?], [$forest-ids as xs:unsignedLong*] ) as element(cts:co-occurrence)*
  Returns value co-occurrences (that is, pairs of values, both of which appear
  in the same fragment) from the specified value lexicon(s).  The
  values are returned as an XML element
   with two children, each child
  containing one of the co-occurring values.  You can use
  cts:frequency
  on each item returned to find how many times the pair occurs.
  Value lexicons are implemented using range indexes; consequently
  this function requires a range index for each input index reference.
  If an index or lexicon is not configured for any of the input references,
  an exception is thrown.
| Parameters | |
|---|---|
| range-index-1 | A reference to a range index. | 
| range-index-2 | A reference to a range index. | 
| options | 
    Options.  The default is ().
     Options include: 
  | 
	    
| query | 
    Only include co-occurrences in fragments selected by the
    cts:query,
    and compute frequencies from this set of included co-occurrences.
    The co-occurrences do not need to match the query, but they must occur in
    fragments selected by the query.
    The fragments are not filtered to ensure they match the query,
    but instead selected in the same manner as
    
    "unfiltered" cts:search
    
    operations.  If a string
   is entered, the string is treated as a cts:word-query of the
   specified string.
   | 
	    
| quality-weight | A document quality weight to use when computing scores. The default is 1.0. | 
| forest-ids | A sequence of IDs of forests to which the search will be constrained. An empty sequence means to search all forests in the database. The default is (). | 
Only one of "frequency-order" or "item-order" may be specified in the options parameter. If neither "frequency-order" nor "item-order" is specified, then the default is "item-order".
Only one of "fragment-frequency" or "item-frequency" may be specified in the options parameter. If neither "fragment-frequency" nor "item-frequency" is specified, then the default is "fragment-frequency".
Only one of "ascending" or "descending" may be specified in the options parameter. If neither "ascending" nor "descending" is specified, then the default is "ascending" if "item-order" is specified, and "descending" if "frequency-order" is specified.
Only one of "eager" or "lazy" may be specified in the options parameter. If neither "eager" nor "lazy" is specified, then the default is "eager" if "frequency-order" or "map" is specified, otherwise "lazy".
Only one of "any", "document", "properties", or "locks" may be specified in the options parameter. If none of "any", "document", "properties", or "locks" are specified and there is a $query parameter, then the default is "document". If there is no $query parameter then the default is "any".
Only one of the "score-logtfidf", "score-logtf", "score-simple", "score-random", or "score-zero" options may be specified in the options parameter. If none of "score-logtfidf", "score-logtf", "score-simple", "score-random", or "score-zero" are specified, then the default is "score-logtfidf".
Only one of the "checked" or "unchecked" options may be specified in the options parameter. If neither "checked" nor "unchecked" are specified, then the default is "checked".
If "collation=URI" is not specified in the options parameter, then the default collation is used. If a lexicon with that collation does not exist, an error is thrown.
If "sample=N" is not specified in the options parameter,
  then all included co-occurrences may be returned.
  If a $query parameter
  is not present, then "sample=N" has no effect.
If "truncate=N" is not specified in the options parameter,
  then co-occurrences from all fragments selected by the
  $query parameter are included.
  If a $query parameter is not present, then
  "truncate=N" has no effect.
To incrementally fetch a subset of the co-occurrences returned by this
  function, use fn:subsequence
  
  on the output, rather than 
  the "skip" option. The "skip" option is based on fragments matching the 
  query parameter (if present), not on occurrences. A fragment 
  matched by query might contain multiple occurrences or no occurrences. 
  The number of fragments skipped does not correspond to the number of 
  values. Also, the skip is applied to the relevance ordered query matches, 
  not to the ordered co-occurrences list. 
When using the "skip" option, use the "truncate" option rather than the "limit" option to control the number of matching fragments from which to draw values.
(: Suppose we have a document with many news items in the database. :)
xdmp:document-insert("/news.xml",
<news>
  <sale>
   <sold-item>Bike</sold-item>
    <name>
      <first>John</first><last>Griffith</last>
    </name>
    <city>Reno</city>
    <text>...</text>
  </sale>
  <sale>
   <sold-item>Car</sold-item>
    <name>
      <first>Will</first><last>Shields</last>
    </name>
    <city>Lexington</city>
    <text>...</text>
  </sale>
  <theft>
    <stolen-item>Bike</stolen-item>
    <reporter>
      <first>John</first><last>Smith</last>
    </reporter>
    <city>Las Vegas</city>
    <text>...</text>
  </theft>
  <theft>
    <stolen-item>Car</stolen-item>
    <reporter>
      <first>Will</first><last>Shields</last>
    </reporter>
    <city>Indianapolis</city>
    <text>...</text>
  </theft>
</news>);
(: Now suppose we have two path range indexes defined.
     Index1: /news/sale/sold-item string type
     Index2: /news/sale/city  string type
   We can find co-occurrence of items sold and the city it was sold in.
:)
  cts:value-co-occurrences(cts:path-reference("/news/sale/sold-item"),
                           cts:path-reference("/news/sale/city"))
=>
<cts:co-occurrence xmlns:cts="http://marklogic.com/cts"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <cts:value xsi:type="xs:string">Bike</cts:value>
  <cts:value xsi:type="xs:string">Lexington</cts:value>
</cts:co-occurrence>
<cts:co-occurrence xmlns:cts="http://marklogic.com/cts"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <cts:value xsi:type="xs:string">Bike</cts:value>
  <cts:value xsi:type="xs:string">Reno</cts:value>
</cts:co-occurrence>
<cts:co-occurrence xmlns:cts="http://marklogic.com/cts"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <cts:value xsi:type="xs:string">Car</cts:value>
  <cts:value xsi:type="xs:string">Lexington</cts:value>
</cts:co-occurrence>
<cts:co-occurrence xmlns:cts="http://marklogic.com/cts"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <cts:value xsi:type="xs:string">Car</cts:value>
  <cts:value xsi:type="xs:string">Reno</cts:value>
</cts:co-occurrence>
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