to manifold 20 the four actuation signals fed to the hydraulic actuator 20 to effect a raising and lowering of the mast and opening and closing of the closure are given that a dot represents the logical 34 and 34 function and a plus represents the logical 34 or 34 an equation can be formulated for the initiation of the lm actuation signal equation a allows the generation of an actuation signal to lower the mast if the closure is open and the operator requests mast retraction or if the closure is not fully open and the mast is not seated all the way down this latter capability is necessary to prevent the mast 39 s creeping up due to hydraulic malfunction or colliding with a closed or partially opened closure in this regard the designation of the bar over a signal designation indicates the absence of the presence of the signal for the purposes of this explanation this condition is referred to as being the 34 not 34 presence of a signal thus in equation a above it is apparent if the command signal lower mast lm is given the indication from mast sensor 16 would be that the mast is now down md since there is no positive indication that the.