[Boost-build] --build-type=complete Seems to Build Too Many Files

Subject: [Boost-build] --build-type=complete Seems to Build Too Many Files
From: Merrill Cornish (merrill.cornish_at_[hidden])
Date: 2015-04-12 21:34:53

I am running Windows 7 64-bit (fully patched), compiling with 
MinGW-4.9.2 64b-bit and building Boost 1.57.0 using

\bin\b2.exe  --prefix=d:\boost_1_57_0  --build-type=complete 
--without-python  --without-mpi
      cxxflags="-std=c++11"  -a toolset=gcc  stage >d:\bin\build.log 2>&1

As I understand it, a "complete" build should build eight variations:
     release/debug, static/dynamic, and single thread/multi-thread

However, when I build, I get 16.  For example (from in stage/lib):

libboost_chrono-mgw49-1_57.a     101KB
libboost_chrono-mgw49-1_57.dll       75KB
libboost_chrono-mgw49-1_57.dll.a     15KB
libboost_chrono-mgw49-d-1_57.a
libboost_chrono-mgw49-d-1_57.dll
libboost_chrono-mgw49-d-1_57.dll.a
libboost_chrono-mgw49-mt-1_57.a
libboost_chrono-mgw49-mt-1_57.dll
libboost_chrono-mgw49-mt-1_57.dll.a
libboost_chrono-mgw49-mt-d-1_57.a
libboost_chrono-mgw49-mt-d-1_57.dll
libboost_chrono-mgw49-mt-d-1_57.dll.a
libboost_chrono-mgw49-mt-s-1_57.a
libboost_chrono-mgw49-mt-sd-1_57.a
libboost_chrono-mgw49-s-1_57.a
libboost_chrono-mgw49-sd-1_57.a

I had thought the  .a  file extension marked a Unix archive (i.e., 
non-shared object).  In reading through the build log, I first thought  
.a  was a Windows object library while  .dll  was the Windows shared 
library--but then what is  .dll.a ?

For reference, I've included the file sizes of the first three.

I expected eight.  I got sixteen.  What are they?

Merrill Cornish