In this paper, we present a case study in deriving an algorithm from a formal specification via FermaT transformations. The general method (which is presented in a separate paper) is extended to a method for deriving an implementation of a program transformation from a specification of the program transformation. We use program slicing as an example transformation since this is of interest outside the program transformation community. We develop a formal specification for program slicing in the form of a WSL specification statement which is refined into a simple slicing algorithm by applying a sequence of general purpose program transformations and refinements. Finally, we show how the same methods can be used to derive an algorithm for semantic slicing. The main novel contributions of this paper are: 1) developing a formal specification for slicing, 2) expressing the definition of slicing in terms of a WSL specification statement, and 3) by applying correctness preserving transformations to the specification, we can derive a simple slicing algorithm.