update of README;

in Makefile, only build tests if required, because it requires ocaml4

Makefile

@@ -6,7 +6,10 @@ LIB = $(addprefix _build/, $(TARGETS))
 INSTALL = $(LIB) sequence.mli
 
 all:
-	ocamlbuild tests.native $(TARGETS) $(DOC)
+	ocamlbuild $(TARGETS) $(DOC)
+
+tests:
+	ocamlbuild tests.native
 
 install: all
 	ocamlfind install $(NAME) META $(INSTALL)

README.md

@@ -2,8 +2,15 @@ Sequence
 ========
 
 Simple sequence abstract datatype, intented to transfer a finite number of
-elements from one data structure to another. It also provides a tiny
-library for S-expressions, convertible to streams of tokens, and conversely.
+elements from one data structure to another. Some transformations on sequences,
+like `filter`, `map`, `take`, `drop` and `append` can be performed before the
+sequence is iterated/folded on.
+
+Sequence is not designed to be as general-purpose or flexible as, say,
+Batteries' `Enum.t`. Rather, it aims at providing a very simple and efficient
+way of iterating on a finite number of values, only allocating (most of the time)
+one intermediate closure to do so. For instance, iterating on keys, or values,
+of a `Hashtbl.t`, without creating a list.
 
 Build
 =====
@@ -13,8 +20,11 @@ You need OCaml, say OCaml 3.12 or OCaml 4.0.
     $ make
 
 To see how to use it, check `tests.ml`. `sequence.ml` has a few examples of how to convert
-data structures into sequences, and conversely. The module `sexpr.mli` exposes the interface
-of the S-expression library.
+data structures into sequences, and conversely.
+
+The module `sexpr.mli` exposes the interface of the S-expression example library. It
+requires OCaml>=4.0 to compile, because of the GADT structure used in the monadic
+parser combinators part of `sexpr.ml`.
 
 License
 =======