question: public void denormalize ( int minLevel , int levelMod , ArrayList < S2CellId > output ) { output . clear ( ) ; output . ensureCapacity ( size ( ) ) ; for ( S2CellId id : this ) { int level = id . level ( ) ; int newLevel = Math . max ( minLevel , level ) ; if ( levelMod > 1 ) { newLevel += ( S2CellId . MAX_LEVEL - ( newLevel - minLevel ) ) % levelMod ; newLevel = Math . min ( S2CellId . MAX_LEVEL , newLevel ) ; } if ( newLevel == level ) { output . add ( id ) ; } else { S2CellId end = id . childEnd ( newLevel ) ; for ( id = id . childBegin ( newLevel ) ; ! id . equals ( end ) ; id = id . next ( ) ) { output . add ( id ) ; } } } }
target:  Replaces "output" with an expanded version of the cell union where any cells whose level is less than "min_level" or where (level - min_level) is not a multiple of "level_mod" are replaced by their children, until either both of these conditions are satisfied or the maximum level is reached. This method allows a covering generated by S2RegionCoverer using min_level() or level_mod() constraints to be stored as a normalized cell union (which allows various geometric computations to be done) and then converted back to the original list of cell ids that satisfies the desired constraints.