diff --git a/tools/nfdc/format-helpers.hpp b/tools/nfdc/format-helpers.hpp
index 6629409..4c7badd 100644
--- a/tools/nfdc/format-helpers.hpp
+++ b/tools/nfdc/format-helpers.hpp
@@ -67,6 +67,16 @@
 
 namespace text {
 
+/** \brief print a number of whitespaces
+ */
+struct Spaces
+{
+  int nSpaces; ///< number of spaces; print nothing if negative
+};
+
+std::ostream&
+operator<<(std::ostream& os, const Spaces& spaces);
+
 /** \brief print different string on first and subsequent usage
  *
  *  \code