Semantic Model (part 1)

Our next step will be to build a semantic model 
of the contents of a Makefile.

A collection of variables
Each var, a list of strings
doggies = dingo.o poodle.o
Or variable references
animals = llama.o $(doggies) loris.o 
Referenced varialbes flatten
        = llama.o dingo.o poodle.o loris.o

A dependency tree
Each node has a target and some dependencies
thezoo: $(animals) $(food) tourism.h
And an action to remake it
gcc -o $@ $(animals) $(food)
An action is a list of shell commands
which are just nested string lists, as with variables