* little-mler/little-mler.sml (LittleMLer): first application of
``plus'' for int arithmetic but get a compiler error.

* little-mler/numbers-by-peano.sig (NUMBERS_BY_PEANO): add the
first signature file for the numbers by Peano.

* little-mler/numbers-by-peano.sig (NumberAsNum): Implement the
very first module for numbers as nums.

* little-mler/plus-over-number.sig: introduce a file for the
signature and the functor for doing arithmetics.

* sexp-expressions/sources.cm: moved the sexp-expressions code in
a dedicated folder with its own sources.cm file for compilation.
* little-mler/numbers-with-reveal-conceal.sig
(NumberAsIntWithRevealConceal): Add a new signature to introduce
two new functions in order to conceal and reveal the type in order
to use ``plus'' with both Int building blocks and Num building
blocks.
* little-mler/little-mler-tests.sml (LittleMLerTests): add two
tests that shows the first way of using ``plus'' with the two sets
of building blocks.

* little-mler/little-mler.sml (LittleMLer): Finish the first way
to use ``plus'' with the two sets of building blocks and reported
the insightful text.