HIV protease inhibitors are disclosed which are characterized as having both hydroxyethylamine and N-heterocyclic moieties. The invention provides a method for identifying string candidates and analyzing the probability that the string candidate matches a user-defined string. The barrel-interacting part of the gas check has one or more external recesses and adjacent radial ridgesedges protruding relative to the recesses, which serves to reduce the total surface area of the gas check contacting the bore. The system further includes a rope length adjustment device which is connectable to the safety rope and adapted to allow the safety rope to pass therethrough so that the effective length of safety rope extending between the rope length adjustment device and the anchor rope can be adjusted up to a certain length determined by the user in order to define a safe working area.