Index: kernel/include/IMP/base_types.h
===================================================================
--- kernel/include/IMP/base_types.h	(revision 400)
+++ kernel/include/IMP/base_types.h	(working copy)
@@ -8,13 +8,13 @@
 #ifndef __IMP_BASE_TYPES_H
 #define __IMP_BASE_TYPES_H
 
+#include "IMP_config.h"
+#include "Index.h"
+
 #include <string>
 #include <iostream>
 #include <vector>
 
-#include "IMP_config.h"
-#include "Index.h"
-
 namespace IMP
 {
 
@@ -67,8 +67,11 @@
 /**
    We need this to have a uniform return type for python.
  */
-typedef std::vector<Particle*> Particles; 
+typedef std::vector<Particle*> Particles;
+typedef std::pair<Particle*, Particle*> ParticlePair;
+typedef std::vector<ParticlePair> ParticlePairs;
 
+
 typedef std::vector<ParticleIndex> ParticleIndexes;
 typedef std::vector<FloatIndex> FloatIndexes;
 typedef std::vector<IntIndex> IntIndexes;