[plt-scheme] suggestions and commendations for exercise 2.21

First of all, convert-gui() and convert-repl() are awesome... now for my 
suggestions:

[1] convert-repl should take an optional argument serving as the escape 
string. This is useful if "x" needs to be tested as input to the program

[2] I had problems trying to use convert-file because I wanted to use a 
filename without specifying an absolute path, but did not know what my 
current working directory was. I therefore did not know where to place 
the input file or where to look for the output file.

At what language level and with what procedure, does one find one's 
current working directory?