-- verifies an evaulator of expressions in PN (polish/prefix notation)
-- expects evaluator as function "solution"
-- returns true upon correct verification

-- assert some stuff for better error messages
assert(log, "log")

function check(output, input)
  local s = tostring(solution(input))
  assert(log, "log")("check "..s)
  if s == output then
    log("OK: "..input.." => "..output)
  else
    log("Oops: "..input.." => "..s.." (expected: "..output..")")
    error("verification failed")
  end
end

check("3", "+ 1 2")
check("6", "* 2 3")

return true