--- a/devel-versioning.rst	Fri Aug 19 00:17:19 2011 +0300
+++ b/devel-versioning.rst	Fri Aug 19 00:52:30 2011 +0300
@@ -15,8 +15,8 @@
  * QA test results (Pass/Fail).
  * Location of full logs.
 
-Versioning formula.
-===================
+Order formula.
+==============
 
 if (A.major != B.major) return A.major > B.major;
 if (A.minor != B.minor) return A.minor > B.minor;
@@ -26,6 +26,27 @@
 if (B.special == "") return -1;
 return A.special > B.special;
 
+**NOTE** Accoding to this definition 1.0.1rc1 < 1.0.1rc10 < 1.0.1rc2 which is
+non meaningful.
+
+Compatibility formula.
+======================
+
+Assume that app linked with new version of lib. Thus
+
+is_compatible_with_old(old, new) {
+  if (old.major != new.major) return 0;
+  if (old.minor > new.minor) return 0;
+  return 1;
+}
+
+Assume that app linked with old version of lib. Thus
+
+is_compatible_with_new(old, new) {
+  if (old.major != new.major) return 0;
+  return 1;
+}
+
 Reference.
 ==========