In any given expression e, a bound variable is one whose binding location is in e, while a free variable is one whose binding location is not in e e2 rightarrow e2' e1 + e2 rightarrow e1 + e2' We observe that the line of the body of the step function pattern matches on input e. In order to capture this inference rule in code we might use the following road map: e1 + e2 The body of the case should implement all store step of e2 to the variable e2' The body of this case returns the expression represented to the of the rightarrow in the