I wish to compile a .cmxa library using ocamlbuild.  I have no trouble 
compiling to a .cma, but for .cmxa, I get the following error:

[awarshaver@nyc-qws-029 multi-oc]$ ocamlbuild janebase/sexp/lib/sexplib.cmxa
+ ocamlfind opt -package bigarray,pcre,res,netstring -I +camlp4 -pp 
camlp4orf -pp 'camlp4o /home/awarshaver/multi-oc/lib/pa_type_conv.cmo 
/home/awarshaver/multi-oc/lib/pa_sexp_conv.cmo' -a -linkpkg -thread 
janebase/sexp/lib/type.cmx janebase/sexp/lib/parser.cmx 
janebase/sexp/lib/lexer.cmx janebase/sexp/lib/sexp.cmx 
janebase/sexp/lib/path.cmx janebase/sexp/lib/conv.cmx 
janebase/sexp/lib/conv_error.cmx -o janebase/sexp/lib/sexplib.cmxa
/usr/local/home/godi310/godi/lib/ocaml/std-lib/unix.cmxa
is not a compilation unit description.
Command exited with code 2.

The only guidance I have found is that there can by no .cmxa files 
included in the library, this is not the case here.  I have a .mllib and 
.mlpack file (I'm not really sure what the difference is).

Any insight would be appreciated.

Andrew