tree.hh |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/tree.hh b/tree.hh
index 1429da3..d780be3 100644
--- a/tree.hh
+++ b/tree.hh
@@ -71,6 +71,7 @@
 #include <set>
 #include <queue>
 #include <algorithm>
+#include <cstddef>
 
 // HP-style construct/destroy have gone from the standard,
 // so here is a copy.