better cleaning in Makefile
changed some indentations
Merge branch 'master' of https://github.com/karlstroetmann/Formal-Languages

* 'master' of https://github.com/karlstroetmann/Formal-Languages:
  fix typos
extend the exercise related to the compiler
use lambda-closures where appropriate

Beginning with setlX v2.3.2, the syntax

  f := x |=> expr;

is supported as an abbreviation for

  f := closure(x) { return expr; };

Various example programs have been changed to make use of this fact.