( ( mod-inv
  =   a b b0 x0 x1 q
    .   !arg:(?a.?b)
      & ( !b:1
        |   (!b.0.1):(?b0.?x0.?x1)
          &   whl
            ' ( !a:>1
              & div$(!a.!b):?q
              & (!b.mod$(!a.!b)):(?a.?b)
              & (!x1+-1*!q*!x0.!x0):(?x0.?x1)
              )
          & (!x:>0|!x1+!b0)
        )
  )
& out$(mod-inv$(42.2017))
};