Yes, PDD 19 can talk about 'subroutines' rather than 'compilation units'. (I just did a quick skim of the file, and a simple search and replace changing 'compilation unit' to 'subroutine' will work fine.)
In imcc.y, change 'compilation_unit' and 'compilation_units' to something more general like 'element', 'component', 'segment', etc... (Not 'statement' but it's basically filling the same purpose as the 'statement' rule in an HLL.)
