are an important special case in which a model is input to such a program.

Some bidirectional languages are ''bijective''. The bijectivity of a language is a severe restriction of its power, because a bijective language is merely relating two different ways to present the very same information.

More general is a lens language, in which there is a distinguished forward direction ("get") that takes a concrete input to an abstract output, discarding some information in the process: the concrete

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