[QBS] What is the semantic difference between Rule and Transformer?

[Smirnov Vladimir]

 From docs:
-A /multiplex rule/ creates one /transformer/ that takes all input 
artifacts with the matching input file tag and creates one or more 
artifacts.
-A /simplex rule/ creates one transformer per matching input file.
-A /transformer/ takes zero or more inputs and produces one or more 
output artifacts from them.

So I can imagine that Rule wraps some Transformer's functionality. But 
why don't just use only Rules? What can i do with Transformer that Rule 
couldn't cover?

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