Logic formulae

Hi, I found below schematic in a theorie book. The given logic formula is: L=a . b . (b + c . d) (where b means b-not)
I highly doubt that the logic formula and schematic correspond.

Is the correct logic formula L=a . b . c . d for given schematic?
Is L=a . b . (b + c . d) incorrect for given schematic?
What is the corrected schematic for L=a . b . (b + c . d) ?