project file syntax

[agent@drrob1.com]

As an exercise for myself, I'm converting an rpn calculator I wrote in
modula-2 to Ada.  I have already debugged several modules, such as
tknrtnsa, timliba and environa.  Now I want to debug 2 modules,
hpcalca and rpna.  I call this project file rpna.gpr

project rpna is
  for Source_Files use ("rpna.adb","hpcalca.ads","hpcala.adb");   for
Source_Dirs use (".", "./**");
  for Main use ("rpna");   

  package Builder is
    for Default_Switches ("Ada")
        use ("-g");
  end Builder;

  package Compiler is
    for Default_Switches ("Ada")
       use ("-fstack-check",
            "-gnatVa",
            "-g",
            "-gnato",
            "-gnatf",
            "-gnatwu",
            "-gnat2012");
  end Compiler;

end rpna;


I get errors that the modules I previously wrote cannot be found. That
is, tknrtnsa, timliba and environa cannot be found.  If I have these
re-compiled by putting them in the Source_Files list, that works.  But
it strikes me as I am not understanding something about project files.

How do I write a project file to not have to recompile packages that I
know already work, and for which I have the .ali and .o files?

Thanks,
Rob