5.3

global [ black]

to start

    ab, setpower 5

    setblack 300

    loop [ if (( sensor 3 ) > black

         and (( sensor 4 ) > black ))

           [ fwd ]

           if (( sensor 3 ) < black

           and (( sensor 4 ) < black ))

           [ r90 ]

           if (( sensor 3 )  < black )

           [ left ]

           if (( sensor 4 ) < black )

           [ right ] ]

end

to fwd

     ab, thisway ab, on

end

to left

    a, thatway b, thisway

    ab, onfor  1

end

to right

    b, thatway a, thisway

    ab, onfor 1

end

to r90

    ab, thisway ab, onfor 2

    a, thatway b, thisway

    ab, nofor 1

end

5.4

global [ black]

to start

    ab, setpower 5

    setblack 300

    loop [ if (( sensor 3 ) > black

         and (( sensor 4 ) > black ))

           [ fwd ]

           if (( sensor 3 ) < black

           and (( sensor 4 ) < black ))

           [ bck ]

           if (( sensor 3 )  < black )

           [ right ]

           if (( sensor 4 ) < black )

           [ left ] ]

end

to fwd

     ab, thisway ab, on

end

to left

    a, thatway b, thisway

    ab, onfor  1

end

to right

    b, thatway a, thisway

    ab, onfor 1

end

to bck

    ab, thatway ab, onfor 2

end