Now let's see if I understand why it works.
Using GreenCard (a module name) instead of GreenCard.lhs (a filename) meant that Hugs added the directory containing GreenCard.lhs (i.e., /home/reid/local/gc-2.01/src) to the list of directories to search in the future so that when it later looked for the module Parse, it (correctly) looked in /home/reid/local/gc-2.01/src
Is that it?
Well, the directory /home/reid/local/gc-2.01/src is in the search path simply because of the -P option; it's just that the search path only plays a role when you demand the loading of a module name. On top of that, Hugs will always extend the search path with the current directory and the directory where the current module was found (which might not be in the serach path proper if the module was specified using an absolute filename). This is important when module A imports some private module B from the same directory, and module A was loaded using its absolute filename. Phew, I guess this is still quite complicated... :-)
[Sorry if I seem a bit thick here - you kept me very well informed of the changes you were making to the search path but I was pretty distracted by my family, returning to Glasgow and starting a (computer consulting) company and didn't pay as much attention as I should have.]
Not a problem at all! BTW, is your preferred email address still reid@cs.utah.edu? -- Johan