public Value get(int index) {
        throw new UnsupportedOperationException(this + " does not support variable lookup by index");
    }
<p>Lookup by index rather than name. This is supported by some optimized
context subclasses.  This default implementation throws
UnsupportedOperationException.</p>

@param index the index of the variable whose value to return.
@return the value of the indexed variable.
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/evaluation/Context.java