[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?

[3] Is convert.ss hardcoded to only work with the Fahrenheit to Celsius 
converter? The Help Desk certainly makes it seem so:

<quote>
The teachpack convert.ss provides three functions:

* convert-gui, which consumes a conversion function from Fahrenheit to Celsius 
and creates a graphical user interface with two rulers, which users can use to 
convert temperatures according to the given temperature conversion function
* convert-repl, which consumes a conversion function from Fahrenheit to Celsius 
and then starts a read-evaluate-print loop. The loop prompts users to enter a 
number and then converts the number according to the given temperature 
conversion function. A user can exit the loop by entering x.
* convert-file; it consumes a file name in, a conversion function from 
Fahrenheit to Celsius, and a string out. The program then reads a number from 
in, converts it according to the given temperature conversion function, and 
prints the result to the newly created file out.
Warning: If out already exists, it is deleted.

</quote>