public static <K, V> Map<K, V> merge(Map<K, V> a, Map<K, V> b) {
    Map<K, V> copy = new HashMap<>(a);
    copy.putAll(b);
    return copy;
  }
Returns a new map with the union of entries from {@code a} and {@code b} where conflicts take the value from
{@code b}.
planetiler-core/src/main/java/com/onthegomap/planetiler/util/MapUtil.java