* Checking filter/filter-out results is incorrect if pattern contains '%'
* Need heuristics to recognize that a variable is local. Propose to use lowercase.
* Need heuristics for the local variable type. Propose '_list' suffix
* Internal source tree has variables in the inherit-product macro argument. Handle it
* Enumerate all environment variables that configuration files use.
* Break mk2rbc.go into multiple files.
* If variable's type is not yet known, try to divine it from the value assigned to it
  (it may be a variable of the known type, or a function result)
* ifneq (,$(VAR)) should translate to
    if getattr(<>, "VAR", <default>):
* Launcher file needs to have same suffix as the rest of the generated files
* Implement $(shell) function
* Write execution tests
* Review all TODOs in mk2rbc.go