copylefted free software. David McGowan has written that there is no reason to believe the GPL could force proprietary software to become free software, but could "try to enjoin the firm from distributing commercially a program that combined with the GPL'd code to form a derivative work, and to recover damages for infringement." If the firm "actually copied code from a GPL'd program, such a suit would be a perfectly ordinary assertion of copyright, which most private firms would defend if the shoe were on the other foot." Richard Stallman has described this view with an analogy, saying, "The GPL's domain does not spread by proximity or contact, only by deliberate inclusion of GPL-covered code in your program. It spreads like a spider plant, not like a virus." Interoperability Popular copyleft licenses, such as the GPL, have a clause allowing components to interact with non-copyleft components as long as the communication is abstract, such as executing a command-line tool with a set of switches or interacting with a Web server. As a consequence, even if one module of an otherwise non-copyleft product is placed under the GPL, it may still be legal for other components to communicate with it normally. This allowed communication may or may not include reusing libraries or routines via dynamic linking — some commentators say it does, the FSF asserts it does not and explicitly adds an exception allowing it in the license for the GNU Classpath re-implementation of the Java library. The interoperability clauses are often pragmatically inoperative due to the vigorous enforcement and strict interpretation of the GPL as it related to integration, aggregation, and linking. It is argued that most forms of incorporation, aggregation, or connectivity with GPL-licensed code is a derivative work that must be licensed under the GPL. In recent years, a number of communities using GPL-incompatible licenses have dropped efforts and support for interoperability with GPL-licensed products in response to this trend. Some developers and communities have switched to the GPL or a GPL-compatible license in response, which critics and supporters alike agree is intentional result. See also Viral phenomenon Permissive license Patentleft References External links MS lawyers join open source fray, CNET, Stephen Shankland, 25 June 2001 Criticism of copyleft