Re: Software in main that is throughly useless without non-free software

[Ben Pfaff <pfaffben@pilot.msu.edu>]

One wrong assumption I keep seeing in this thread is that if you can
read a format, then you can write it.  That's simply not true.  You
may not have enough information.

For instance, suppose that a file format contains a checksum, with the
calculation of the checksum undocumented.  In that case, you will be
able to read the file, although you will not be able to verify that
the checksum is correct.  You will not be able to write a correct
file, since you don't have enough information.

Therefore, the existence of a free program to read a file format does
not imply that a program can be written to write that file format in a
way acceptable to the non-free program that originated the format.  It
does imply, I think, that you can write a free program to write the
file format in a way acceptable to the free program that reads the
file format.

I'm not sure that this has any bearing on the issue, but I think it's
worth mentioning anyway.