procedure teq(a1,a2)
   local i
   if *a1 ~= *a2 then fail	# check sizes
   if a1[1] ~== a2[1] then fail	# check values
   every i := 2 to *a1 do	# check subtrees
      if not teq(a1[i],a2[i]) then fail
   return a2
end