guava/src/com/google/common/io/MoreFiles.java
private static @Nullable Collection<IOException> concat(
      @Nullable Collection<IOException> exceptions, @Nullable Collection<IOException> other) {
    if (exceptions == null) {
      return other;
    } else if (other != null) {
      exceptions.addAll(other);
    }
    return exceptions;
  }
private static @Nullable Collection<IOException> concat(
      @Nullable Collection<IOException> exceptions, @Nullable Collection<IOException> other) {
    if (exceptions == null) {
      return other;
    } else if (other != null) {
      exceptions.addAll(other);
    }
    return exceptions;
  }
Concatenates the contents of the two given collections of exceptions. If either collection is
null, the other collection is returned. Otherwise, the elements of {@code other} are added to
{@code exceptions} and {@code exceptions} is returned.