Summarize the following:
Especially on the harder difficulties, melee weapons will only get you killed. Instead, stay back and pick off enemies from afar. Use your shoves to keep some distance, and, whenever you're in the open, use a pistol or rifle to thin the herd before they get too close. Left 4 Dead is a constantly shifting game, and Infected will appear in unexpected places. Zombies spawn at various points, and they are on a timer. Special Infected like Hunters and Smokers can appear anywhere but usually, come with a horde of Infected to cause maximum damage. There are, however, ways to predict when enemies are coming: Pay attention to the music -- it will start to swell and get tense as enemies build up out of sight. Turn on subtitles. You can often read things like, "coughing sounds" or "noises of the horde" in advance, making it easier to predict incoming groups or specials. Know the three stages of an attack. Build-up, as the music swells. Peak, when the big group descends on you, and Rest, after you've defeated them and have 1-2 minutes to relax. When in crescendo events, or if you just need to catch your breath, choose spots with limited entryways (like rooms with only 1-2 doors or windows) or high areas where you force the enemy to bottleneck. Bullets pierce enemies and hit the Infected behind them, meaning you can mow down large groups quickly and conserve ammunition. Shooting down stairs, ladders, and in doorways can make big crowds much, much easier to handle. Still, keep moving when possible. Holing up for too long drains supplies and can leave you open to attacks by Boomers and Spitters. Ammo is your lifeblood, so don't go draining it. When enemies are at a distance or are easy to deal with, switch to a pistol or melee weapon. Running out of shotgun shells during an intense moment can cost you your life. Medkits are valuable and should be used as such. After your first knockdown, your vision becomes monochromatic (loses color), and you move slower. This is the time to use a Medkit. Otherwise, stick to pain pills and adrenaline whenever possible. If you're approaching a crescendo event and have less than 40 health, now is the time to use your Medkit. Crouching increases aim and is essential if firing at incoming enemies. Whenever you can, crouch and fire, then get up and move as you go. In muddy areas, like "The Parish," you actually move just as fast crouching as standing, so the benefit in accuracy has no trade-off. In the mud and swamps, crouch the entire time. In Versus mode, four players get the chance to play as Smokers, Boomers, Hunters, and more while fighting against four other human players. Playing as Infected is practically its own game, but the best advice is to think about all the ways you've died as a human. What sorts of attacks stranded you from teammates, caught you off-guard, or otherwise dealt more damage than you could handle? For the best results, use your teammates to coordinate attacks together. A Boomer can vomit, bringing in zombies that allow a good Spitter to trap pressured players in a corner, for example. Use surprise to your advantage. Let them run to you and hit them when they're distracted.

summary: Keep your distance whenever possible. Predict the "flow" of the hordes by learning how they spawn. Use choke-points and high ground to your advantage. Conserve ammunition whenever possible. Don't use medkits until necessary, usually after incapacitation. Crouch whenever possible. Know the strengths of each Infected on Versus mode.

Summarize the following:
Put your weight on the stirrup, not on your arms. If you're using a western saddle, your hand can rest the pommel for balance, but do not grab it tightly or pull yourself up with it. As your foot hits the stirrup, the horse will sense the shift in weight. Many poorly trained horses will move away from the weight of your body at this point. Traditionally, horses are always mounted and dismounted from the left side, regardless of whether you are right- or left-handed. When your horse is completely still, and with your foot in the stirrup, start to move your right leg over its back. If your horse starts to move while you are doing so, dismount completely and make it stop again. Don't try to mount anyway as the animal walks out from under you. This is dangerous and encourages bad habits. If the horse allows you to mount, you're ready to begin riding. If the horse begins to move away, immediately remove your foot and take up the slack in the reins. This will teach the horse that you won't want it to move once it feels your weight in the stirrup. Trying to mount while the horse is still moving will teach them that they do not have to stand still. If you need to move the horse back to the starting position, apply pressure on the animal's chest to send it moving backward. You can also keep the horse from moving away by taking up slack from the reins. Alternately, if the horse tries to walk off, keep the reins short so the horse is forced to walk in a small circle. Wait until the horse stops moving, then try putting your foot in the stirrup again.

summary: Place your left foot in the stirrup on the left side of the horse. Step into the stirrup and swing your right leg over the horse's back. Stop mounting immediately if your horse moves away. Push the horse back if it walks away from you as you mount.

Summarize the following:
Many air purifiers use non-washable HEPA filters. Turn off and unplug your appliance before accessing the filter. Check your product manual for specific instructions on how to access your appliance's filter. At least one foam or activated charcoal filter usually accompanies a non-washable HEPA filter. These accompanying filters typically need to be rinsed for two to three minutes, or until water runs clear. Towel dry your foam or activated charcoal filters, then let them air dry completely for at least 24 hours. Use your vacuum cleaner's hose with a nozzle or brush attachment to clean your non-washable HEPA filter. Run the attachment over the filter until you've removed all debris. Take care not to puncture the filter with the vacuum attachment. Reassemble your appliance after the washable filters have dried. You can wrap the HEPA filter tightly in plastic while you're waiting for the other filters to dry or during any other extended period of non-use. Some air purifiers have an electronic filter clean reminder. If yours has one, reset it after cleaning your filter.
summary: Remove the filter from your appliance. Wash your appliance's other filters. Run a vacuum cleaner hose attachment over the filter. Reassemble the appliance.