### `++need`

Unwrap unit

Retrieve the value from a `++unit` and crash if the unit is null.

Accepts
-------

`a` is a unit.

Produces
--------

Either the unwrapped value of `a` (`u.a`), or crash.

Source
------

    ++  need                                                ::  demand
      |*  a/(unit)
      ?~  a  ~|(%need !!)
      u.a


Examples
--------

    ~zod/try=> =a ((unit [@t @t]) [~ ['a' 'b']])
    ~zod/try=> (need a)
    ['a' 'b']
    ~zod/try=> =a ((unit @ud) [~ 17])
    ~zod/try=> (need a)
    17
    ~zod/try=> =a ((unit @) [~])
    ~zod/try=> (need a)
    ! exit



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