//  assume a document created with:
//    declareUpdate();
//    xdmp.documentInsert("/example.json",
//    {"a":"aa","b":"bb"});
//
declareUpdate();
for (var x of fn.doc("/example.json") )
xdmp.nodeDelete(x.root.b);
=>
 /example.json now looks like this:
 {"a":"aa"}