to bsearch :value :a :lower :upper
  if :upper < :lower [output []]
  localmake "mid int (:lower + :upper) / 2
  if item :mid :a > :value [output bsearch :value :a :lower :mid-1]
  if item :mid :a < :value [output bsearch :value :a :mid+1 :upper]
  output :mid
end