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<theory, reduction> (HNF) A term describing a lambda expression whose top level is either a variable, a data value, a built-in function applied to too few arguments, or a lambda abstraction whose body is not reducible.

I.e. the top level is neither a redex nor a lambda abstraction with a reducible body.

An expression in HNF may contain redexes in argument postions whereas a normal form may not.

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