The blind and the lame

Once upon a time, there was a blind man and a lame man. The blind man rides a bicycle to drive the lame man. If the blind can't see the road, the lame have to watch the road. The lame man saw a foot in front of him and said, "gogogo". Then the blind man went on shouting, "Euler, Euler, Euler". Then they fell into the ditch together.