### `++sub`

Subtract

Subtracts `b` from `a`.

Accepts
-------

`a` is an atom.

`b` is an atom.

Produces
--------

An atom.

Source
------

        ++  sub                                                 ::  subtract
          ~/  %sub
          |=  [a=@ b=@]
          ~|  %subtract-underflow
          ^-  @
          ?:  =(0 b)  a
          $(a (dec a), b (dec b))

Examples
--------

    ~zod/try=> (sub 10 5)
    5
    ~zod/try=> (sub 243 44)
    199
    ~zod/try=> (sub 5 0)
    5
    ~zod/try=> (sub 0 5)
    ! subtract-underflow
    ! exit



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