NSObject
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+---MTChainFactory
This factory can instantiate objects of the class MTChain. If you want to create subclasses of MTChain, implement a subclass of MTChainFactory and overwrite +newInstance. Then set it to be the new default factory class with +setDefaultChainFactory:.
+(id)newChainWithCode:(char)code
helper
create instance of target class
+(void)setDefaultChainFactory:(Class)klass
set class which can instantiate target class