private FieldExpression on0(int n) {
    if (n == 0) {
      return always();
    } else {
      return on(n);
    }
  }
Composes an overlay object.

Output is a flattened view of the input objects.
having the value from the first one which has it defined from the overlays.
ql/src/java/org/apache/hadoop/hive/ql/parse/ScheduledQueryAnalyzer.java