how would I go about debugging a ports dependency problem?

[Brandon S. Allbery KF8NH]

I'm building a test 5.2-RELEASE box, and installing (most of) the ports
that I have installed on my 4.9-STABLE laptop.  In the process of this,
I discovered that security/nss won't install because of a path conflict
with converters/btoa... yet my laptop has btoa-5.2.1_1,1 and nss-3.8
installed (the same versions the 5.2 machine wants to install).

How do I go about figuring out how my laptop installed them without
conflicts, while the desktop complains?  How likely is it that the
conflict is specifically because one is 4.9 and the other 5.2?  I can
see some angles of attack, but am wondering if there's better ways to do
it.

-- 
brandon s. allbery    [linux,solaris,freebsd,perl]     allbery at kf8nh.com
system administrator      [WAY too many hats]        allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon univ.         KF8NH