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That makes it a very good embeddable program, but then I do like Free Software.

Why would this be bad? If you are developing commercial software you want to keep closed-source, look for a commercial closed-source license.

IIRC, there is another license (guile, maybe?) that's identical to GPL except it doesn't affect the outer program.

MIT license is far less restrictive.

Of course, but if you want to go with the GPL, you can use exceptions and protect just the part you want.

That's the LGPL, used in the GMP library for example.

You're right - Guile has its own license. The LGPL has similar concessions to linking to non-GPL programs, though.