Since the Metalib API is not public, to my knowledge, I don't know whether it gets disclosed with an NDA. And you can't run or develop Xerxes without an ExLibris License, because it depends on a proprietary and unspecified data set.
I'm sure that's legal, but it's not true to the spirit of copyleft. The main effect of using GPL for Xerxes is that it prevents ExLibris from distributing (but not using) proprietary versions of Xerces. If that is the intent of the developers, then perhaps AGPL would be a better tool for them to wield.
None of this should be taken as a criticism of the Xerxes developers.
On Feb 18, 2011, at 3:50 AM, graham wrote:
> That's very different from saying something with a GPL license can't use
> a proprietary interface. As if for example Xerxes couldn't use the
> Metalib API - without which it would be pointless. As I understand him
> Eric is saying that there are interfaces to library software which
> actually have a license or contract which blocks GPLed software from
> using them. It would be a kind of 'viral BSD' license, killing free
> software (in the FSF sense) but leaving proprietary or open source (in
> your Apache/MIT sense) untouched. I haven't seen any examples myself,
> and can't quite see how it would be done legally.
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