What makes a StableInterface?

''please add -- PeterSommerlad''

	* it is supported by a GreatAbstraction
	* it incorporates DesignForSlippage
	* it is easy to use
	* it avoids too strong typing (TypeSafeInterfacesConsideredHarmful)
	* it provides good default behavior, even if it is misused
	* it doesn't leak implementation details, such as data structures
	* it can serve implementations and clients far beyond its initial intention
	* everybody understands it and its limitations
	* it was invented by its users and used by its inventors
	* it is usually a result of rigorous refactoring or designed by very great practical thinkers that are thinking practitioners
	* it lacks any really bad features, so no-one feels driven to fix them
	* it lacks any really obvious gaps or missing features, so no-one feels driven to add them
	* it follows the LawOfDemeter

Tongue in cheek answers:
	* It's too hard to change
	* Nobody's using it, so no need to change
	* It is not permitted to change, everyone codes work arounds to use it

What else makes a StableInterface an AbsoluteJoyToUse? -- DavidBennett
	* it can be queried for additional services or extensions.  (This PatLet is used in Unix file I/O, X11, OpenGL, COM, CORBA etc.) In PoSaTwo this is called ExtensionInterface (aka ExtensionObject).
	* it can be queried for information about itself (eg COM/TLB, .NET)

Can we make a pattern from this stuff? 

''I think that a StableInterface is a synergistic result of several patterns'' --NatPryce.
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But is a stable interface always worth the cost?
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see LeastFlexibleProtocolWins, ImplementationIndependenceLimits