[ilyak]

"If this is true and everything on the language are built atop of those, or atop of stuff built atop of those (in n-levels), how can different implementations possible be incompatible?" You cannot build i/o atop of axioms. It would always be a foreign thing that you have to deal with, incompatibly of using a long spec.

[rgoddard]

Details matter, and ultimately the code needs to run on a machine. Hence the implementation matters.

[CyberFonic]

Implementation is where theory and practice collide in a conflict! In GNU tradition, guile (a Scheme) is widely available. Personally, I've found newLisp (http://www.newlisp.org/) to be small, fast, portable across all main platforms, good if you want to deploy web-apps, not so good if you want to write full-featured GUI apps.

As for the Linux comparison, once you really know (i.e. can use command line, the shells, awk, grep, sed, etc) one Linux distro, you can work your way through the others and even AIX, the various BSDs, HPUX, Mac OS/X and Solaris.