L is a finite set of characters called the input alphabet .
F is a subset of K and is called the set of final or accepting states . It is not difficult to see that if we take the upper path from the start state we can accept strings like b, ab, aab, aaab, etc. And if we take the lower path we can accept strings like b, bc, bcc, bccc, etc. This fact is clear from observing that any variable in an expression can be replaced with the expression representing the value of the variable. For any expression, this replacement process can be continued until there are no more variables.