//   This query assumes range indexes of type unsignedLong
//   with positions are configured in the database for both
//   xval and yval. It generates some sample data and then
//   performs the aggregation in a separate transaction.

//  Generate data:
declareUpdate();
 
for (x=1; x<11; x++) {
  var j = 2 * x + 1;
  var uri = fn.concat(x, ".json");
  var vals = {xval:x, yval:j}
  var obj = {a:vals};
  xdmp.documentInsert(uri, obj);
  };

*******
// Run the following query:

cts.linearModel(
    [cts.jsonPropertyReference("yval", ["type=unsignedLong"]),
     cts.jsonPropertyReference("xval", ["type=unsignedLong"])],
    "item-frequency");

=>
math.linearModel(
  <math:linear-model intercept="0.352941176470588" 
      coefficients="1.97058823529412" rsquared="0.995196571090748" 
      xmlns:xs="http://www.w3.org/2001/XMLSchema" 
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xmlns:math="http://marklogic.com/xdmp/math"/>)