The first phase of compilation involves compiling the Ppml program
into a Le-Lisp module. This is done by clicking on the
button of the Ppml pulldown added by the Ppml
environment. If your program contains any errors, a window opens up to
display them.
Once the Ppml file has been compiled, we can display Exp trees
with it. If an abstract tree for a program written in Exp exists
already in another editor, selecting 
in the
pulldown will have Centaur load the basic pretty
printer and recompute the tree display. While developing a pretty
printer, we recommend using two editors: one containing the pretty
printer specification for your language and one containing an actual
program written in the new language. You can thus very easily modify
the Ppml file interactively, recompile it, and see the results.