public Grouping setForceSinglePass(boolean forceSinglePass) {
        this.forceSinglePass = forceSinglePass;
        return this;
    }
Sets whether or not grouping should be forced to execute in a single pass.
If false, this <code>Grouping</code>
might still execute in a single pass due to other constraints.

@param forceSinglePass true to force execution in single pass
@return this, to allow chaining
searchlib/src/main/java/com/yahoo/searchlib/aggregation/Grouping.java