@@ can I have different strategies for src/out? Would I be able 
   to come up with generic implicit rules that work for all cases? 

@@ optional facilities could go into pre-load.

@@ from building point of view distinction on source and generated
   files is not essential.

@@ Common places for generated files (from multi-arch):

   - home (out == src)

   - parallel structure outside (out is set based on curdir?)

   - subdirs in source (potentially many) (out := src + something)