trip(To) --> trip(0,To).

trip(Here,Here) --> [].

trip(From,To) -->
   step(HowMuch),
   {NewFrom is From + HowMuch},
   trip(NewFrom,To).


step(1) --> [forward].

step(-1) --> [back].

step(0) --> [think].

/*********************
?- phrase(trip(N), [forward]).

?- phrase(trip(N), [forward,forward,think,back,think,forward,forward]).

**********/