diff --git a/Modules/mathstuff2.f90 b/Modules/mathstuff2.f90
index 2536ae6..afa6c52 100644
--- a/Modules/mathstuff2.f90
+++ b/Modules/mathstuff2.f90
@@ -28,6 +28,25 @@ module mathstuff2
 
   end subroutine 
 
+  ! ----------------------------------------------------------------
+
+logical function diff_sign(a, b)
+   integer, intent(in)     :: a, b
+
+   ! write(0, *) "diff_sign", a, b
+   if ( (a .lt. 0) .and. (b .ge. 0) ) then 
+      diff_sign = .TRUE.
+      return
+   endif
+   if ( (a .ge. 0) .and. (b .lt. 0) ) then
+      diff_sign = .TRUE.
+      return
+   endif
+   diff_sign = .FALSE.
+
+end function
+
+
   ! ----------------------------------------------------------------
 end module mathstuff2