diff --git a/histogram.sublime-project b/histogram.sublime-project
index 764eccd4..d07b9a25 100644
--- a/histogram.sublime-project
+++ b/histogram.sublime-project
@@ -10,10 +10,10 @@
 	{
 		"ClangFormat":
 		{
-		    "style": "File",
+		  "style": "File",
 			"format_on_save": true
 		},
-		"trim_trailing_white_space_on_save": false,
+		"trim_trailing_white_space_on_save": true,
 		"ensure_newline_at_eof_on_save": false,
 	},
 	"build_systems":
diff --git a/include/boost/histogram/algorithm/project.hpp b/include/boost/histogram/algorithm/project.hpp
index db436d2d..532392b8 100644
--- a/include/boost/histogram/algorithm/project.hpp
+++ b/include/boost/histogram/algorithm/project.hpp
@@ -12,6 +12,7 @@
 #include <boost/assert.hpp>
 #include <boost/histogram/detail/axes.hpp>
 #include <boost/histogram/detail/index_mapper.hpp>
+#include <boost/histogram/detail/is_set.hpp>
 #include <boost/histogram/histogram_fwd.hpp>
 #include <boost/histogram/unsafe_access.hpp>
 #include <boost/mp11.hpp>
@@ -22,11 +23,19 @@ namespace boost {
 namespace histogram {
 namespace algorithm {
 
-/// Returns a lower-dimensional histogram
+// TODO: make generic reduce, which can sum over axes, shrink, rebin
+
+/**
+  Returns a lower-dimensional histogram, summing over removed axes.
+
+  Arguments are the source histogram and compile-time numbers, representing the indices of
+  axes that are kept. Returns a new histogram which only contains the subset of axes.
+  The source histogram is summed over the removed axes.
+*/
 template <typename A, typename S, std::size_t I, typename... Ns>
 auto project(const histogram<A, S>& h, mp11::mp_size_t<I> n, Ns... ns) {
-  // TODO: check that n's are unique
   using LN = mp11::mp_list<mp11::mp_size_t<I>, Ns...>;
+  static_assert(mp11::mp_is_set<LN>::value, "indices must be unique");
 
   const auto& axes = unsafe_access::axes(h);
   auto r_axes = detail::make_sub_axes(axes, n, ns...);
@@ -61,12 +70,16 @@ auto project(const histogram<A, S>& h, mp11::mp_size_t<I> n, Ns... ns) {
   return r_h;
 }
 
-/// Returns a lower-dimensional histogram
+/**
+  Returns a lower-dimensional histogram, summing over removed axes.
+
+  This version accepts an iterator range that represents the indices which are kept.
+*/
 template <typename A, typename S, typename Iterator,
           typename = detail::requires_axis_vector<A>,
           typename = detail::requires_iterator<Iterator>>
 auto project(const histogram<A, S>& h, Iterator begin, Iterator end) {
-  // TODO: check that n's are unique
+  BOOST_ASSERT_MSG(detail::is_set(begin, end), "indices must be unique");
   using H = histogram<A, S>;
 
   const auto& axes = unsafe_access::axes(h);
diff --git a/include/boost/histogram/detail/index_mapper.hpp b/include/boost/histogram/detail/index_mapper.hpp
index a3fcb179..05752cc5 100644
--- a/include/boost/histogram/detail/index_mapper.hpp
+++ b/include/boost/histogram/detail/index_mapper.hpp
@@ -7,17 +7,20 @@
 #ifndef BOOST_HISTOGRAM_DETAIL_INDEX_MAPPER_HPP
 #define BOOST_HISTOGRAM_DETAIL_INDEX_MAPPER_HPP
 
-#include <array>
+#include <boost/container/static_vector.hpp>
+#include <boost/histogram/histogram_fwd.hpp>
 #include <cstddef>
 
 namespace boost {
 namespace histogram {
 namespace detail {
-class index_mapper : public std::array<std::pair<std::size_t, std::size_t>, 32> {
+class index_mapper
+    : public boost::container::static_vector<std::pair<std::size_t, std::size_t>,
+                                             axis::limit> {
 public:
   std::size_t first = 0, second = 0, ntotal = 1;
 
-  index_mapper(std::size_t n) : dims_end(begin() + n) {}
+  using static_vector<std::pair<std::size_t, std::size_t>, axis::limit>::static_vector;
 
   bool next() {
     ++first;
@@ -31,11 +34,6 @@ public:
     }
     return first < ntotal;
   }
-
-  iterator end() { return dims_end; }
-
-private:
-  iterator dims_end;
 };
 } // namespace detail
 } // namespace histogram
diff --git a/include/boost/histogram/detail/is_set.hpp b/include/boost/histogram/detail/is_set.hpp
new file mode 100644
index 00000000..182b8b15
--- /dev/null
+++ b/include/boost/histogram/detail/is_set.hpp
@@ -0,0 +1,30 @@
+// Copyright 2018 Hans Dembinski
+//
+// Distributed under the Boost Software License, Version 1.0.
+// (See accompanying file LICENSE_1_0.txt
+// or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#ifndef BOOST_HISTOGRAM_DETAIL_IS_SET_HPP
+#define BOOST_HISTOGRAM_DETAIL_IS_SET_HPP
+
+#include <algorithm>
+#include <boost/container/static_vector.hpp>
+#include <boost/histogram/detail/meta.hpp>
+#include <boost/histogram/histogram_fwd.hpp>
+
+namespace boost {
+namespace histogram {
+namespace detail {
+template <class Iterator>
+bool is_set(Iterator begin, Iterator end) {
+  using T = iterator_value_type<Iterator>;
+  boost::container::static_vector<T, axis::limit> v(begin, end);
+  std::sort(v.begin(), v.end());
+  auto end2 = std::unique(v.begin(), v.end());
+  return static_cast<std::size_t>(std::distance(v.begin(), end2)) == v.size();
+}
+} // namespace detail
+} // namespace histogram
+} // namespace boost
+
+#endif
diff --git a/include/boost/histogram/histogram_fwd.hpp b/include/boost/histogram/histogram_fwd.hpp
index e2f969ca..2c0cb72c 100644
--- a/include/boost/histogram/histogram_fwd.hpp
+++ b/include/boost/histogram/histogram_fwd.hpp
@@ -7,12 +7,23 @@
 #ifndef BOOST_HISTOGRAM_HISTOGRAM_FWD_HPP
 #define BOOST_HISTOGRAM_HISTOGRAM_FWD_HPP
 
+#include <boost/config.hpp>
 #include <boost/container/container_fwd.hpp>
 
 namespace boost {
 namespace histogram {
 namespace axis {
 
+/* Most of the histogram code is generic and works for any number of axes. Buffers with a
+ * fixed maximum capacity are used in some places, which have a size equal to the rank of
+ * a histogram. The buffers are statically allocated to improve performance, which means
+ * that they need a preset maximum capacity. 48 seems like a safe upper limit for the rank
+ * (you can nevertheless increase it here if necessary): the simplest non-trivial axis has
+ * 2 bins; even if counters are used which need only a byte of storage per bin, this still
+ * corresponds to 256 TB of storage.
+ */
+BOOST_ATTRIBUTE_UNUSED static constexpr unsigned limit = 48;
+
 struct null_type {}; /// empty meta data type
 
 enum class option_type {
diff --git a/include/boost/histogram/storage_adaptor.hpp b/include/boost/histogram/storage_adaptor.hpp
index 983aa032..86768cf3 100644
--- a/include/boost/histogram/storage_adaptor.hpp
+++ b/include/boost/histogram/storage_adaptor.hpp
@@ -282,7 +282,7 @@ struct storage_adaptor : detail::storage_augmentation<T> {
 
   storage_adaptor& operator/=(const double x) { return operator*=(1.0 / x); }
 
-  template <typename U, typename = detail::requires_storage<U>>
+  template <typename U>
   bool operator==(const U& u) const {
     const auto n = this->size();
     if (n != u.size()) return false;