Hi,
I'm trying to make this program work :)
we have a list common([a,d,a,c,d,a,b],R). and R=a the most common letter!
so this is how we probably should find it:

common([X|T],E):-  last(T,E),%find the last element (b)
                sum([X|T],E,M1),%find how many times that element repeats itself ,count b M1=1
               sum([X|T],X,M2),%compare first element of the list with the rest get count(lets say a repeats itself 3 times [a,a,b,a] but sum  only works with the first element of the list and leaves the others)
              M2>M1->(M1 is M2 ; change(X,E));    %because a count is 3 we change E to a! and change M1 value to M2 value(something is not right here :/)
            common(T,E).         
common([_|T],E).

last([Elem], Elem).%last list elem
last([_|Tail], Elem) :- last(Tail, Elem).

change(X,E):-append([],X,E).

sum([X|T],E,M1):- X = E,sum(T,E,C),M1 is 1+C.% count how many times repeats itself
sum([X|T],E,M1):- X \== E,sum(T,E,M1).
sum([],E,0).