Clarify the documentation of IndexSet::is_empty().

diff --git a/include/deal.II/base/index_set.h b/include/deal.II/base/index_set.h
index 8cc267ebbae0413132bdf2a40fa4371e223081ed..9605490868ed981bf42c2c689498704e494b38d9 100644 (file)
--- a/include/deal.II/base/index_set.h
+++ b/include/deal.II/base/index_set.h
@@ -255,6 +255,11 @@ public:
   /**
    * Return whether the index set stored by this object contains no elements.
    * This is similar, but faster than checking <code>n_elements() == 0</code>.
+   *
+   * Note that a set being empty does not imply that the size of the
+   * index space must be zero. Rather, this function returns `true` if
+   * the subset of indices in the index space is the empty set,
+   * regardless of the size of the index space.
    */
   bool
   is_empty() const;