A party walk is a sequence of consequent steps, whose direction with respect to the starting point is either forward (abbr. f), backward (b), left (1) or right (r). For instance, the sequence fibbrrfrff describes the following walk, whose total result is to move the walker a distance of two steps forward (and concurrently two steps to the right): Design a context-free grammar that generates all party walks whose total result is to move the walker at least one step forward from the starting point (ignoring any possible sideways movement). A party walk is a sequence of consequent steps, whose direction with respect to the starting point is either forward (abbr. f), backward (b), left (1) or right (r). For instance, the sequence fibbrrfrff describes the following walk, whose total result is to move the walker a distance of two steps forward (and concurrently two steps to the right): Design a context-free grammar that generates all party walks whose total result is to move the walker at least one step forward from the starting point (ignoring any possible sideways movement)