Refactored a few things + added types.

lib/bppsuite.ml

@@ -25,7 +25,7 @@ let conf_file ~tree ~nb_sites =
            |}
   ]
 
-let bppseqgen ~tree ~nb_sites = workflow ~descr:"bppsuite.bppseqgen" [
+let bppseqgen ~nb_sites ~tree = workflow ~descr:"bppsuite.bppseqgen" [
     cmd "bppseqgen" ~env [
       assign "param" (file_dump (conf_file ~tree ~nb_sites)) ;
     ]

lib/bppsuite.mli

+open Bistro.Std
+open Bistro_bioinfo.Std
+open File_formats
+
+val bppseqgen :
+  nb_sites:int ->
+  tree:nhx workflow ->
+  (* nucleotide_ *)fasta workflow

lib/file_formats.ml

+open Bistro.Std
+open Bistro_bioinfo.Std
+
+class type nhx = object
+  inherit text_file
+  method format : [`nhx]
+end
+
+(* type nucleotide_fasta = < fasta ; alphabet : [`Nucleotide] > *)
+
+