public Set<String> names() {
        throw new UnsupportedOperationException(this + " does not support return a list of its names");
    }
Returns all the names available in this, or throws an
UnsupportedOperationException if this operation is not supported. This
default implementation does the latter.

@return the set of all variable names.
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/evaluation/Context.java