Method ceiling

guava/src/com/google/common/collect/Maps.java:4007–4010

(@ParametricNullness K e)

    }

    @Override
    public @Nullable K ceiling(@ParametricNullness K e) {
      return map().ceilingKey(e);
    }

    @Override
    public @Nullable K higher(@ParametricNullness K e) {