write start 1
read start 1
write end 1 returning 0
read end 1
write start 2a
read start 2
write end 2a returning 1
write start 2b
write end 2b returning 0
read end 2
write start 3pre
write end 3pre returning 1
read start 3
write start 3
write end 3 returning 0
read end 3
write start 4pre
write end 4pre returning 1
read start 4
write start 4a
write end 4a returning 1
write start 4b
write end 4b returning 0
read end 4
read start 5
read end 5