The definition of a rule is  an official regulation, code of regulations or set practice. An example of a rule is that a red light means stop. An example of a rule is an employer demanding their employees arrive at 8am.
New injectors cost anywhere from $80-$120 apiece, making their replacement a somewhat pricey endeavor. There are three types of injector leaks-it can leak fuel into the manifold from the nozzle, it can leak fuel into the engine compartment from the fuel rail, and it can leak air (vacuum leak) from the manifold.
BMW engines are using technologies such as: Direct injection (called High Precision Injection by BMW). Engines including the turbocharged N54 and naturally-aspirated N53 (mostly European markets) used the more costly piezo injectors. However, where lean burn cannot be used (such as in North America), less costly solenoid-type injectors are often used- for example in the N54's successor, the N55.