function Move(direction, steps)
    if direction == "frwd" then
        for i= 1, steps, 1 do
            turtle.forward()
        end
    elseif direction == "back" then
        for i= 1, steps, 1 do
            turtle.back()
        end
    elseif direction == "up" then
        for i = 1, steps, 1 do
            turtle.up()
        end
    elseif direction == "down" then
        for i = 1, steps, 1 do
            turtle.down()
        end
    else
        error("err: invalid direction")
    end
end

function Turn(direction, steps)
    if direction == "left" then
        for i = 1, steps, 1 do
            turtle.turnLeft()
        end
    elseif direction == "right" then
        for i = 1, steps, 1 do
            turtle.turnRight()
        end
    else
        error("err: invalid direction")
    end
end

local modem = peripheral.wrap("left")
modem.open(1)

while true do
    local event,
    modemSide,
    senderChannel,
    replyChannel,
    message,
    senderDistance = os.pullEvent("modem_message")

    if message == "end" then
        break
    elseif message == "frwd" then
        Move("frwd", 1)
    elseif message == "back" then
        Move("back", 1)
    else
        error("err: invalid command")
    end
end