[plt-scheme] Square braces in MrEd

Hi,

I'm still in the middle of writing my first non-trivial Scheme programs
but I am a bit confused at this point. The PLT documentation sometimes
uses [ square braces ] in the code (e.g. command-line) whereas MrEd
only lets me enter ( parentheses ). It looks like they are doing just
the same - that's why I ask what the difference between them is and
what to use.

regards,
Marek

P.S.: It would be nice, if the COMMAND-LINE example would show also
setting a value to a flag instead of just #t. It took me a bit of time
to figure out that there can be an optional id specified. Otherwise the
example is very helpful.