This class represents an NA strand; it is abstract because we don$$t know which bases a strand allows unless we know whether it is RNA or DNA. However, a lot of the behaviour of an NAStrand can be implemented without knowing the exact bases. Your task is to fill in the constructors and methods according to the specifications below, using implementations based on those discussed in lecture. The provided implementations are incorrect, but should be enough to compile and run the $($failing$)$ local and autograder tests.