notbeginswith(_, []).
notbeginswith(IE,[OE|_]) :- IE \= OE.

rle([], []).
rle([[1,OE]|IT], [OE|OT]) :- rle(IT, OT), notbeginswith(OE, OT).
rle([[N1,E]|IT], [E|OT]) :- notbeginswith(E, IT), rle([[N,E]|IT], OT), N1 is N+1.

% rle([[2,a]],[a,a]).
% rle([[1,a],[2,b],[3,c]],[a,b,b,c,c,c]).
% rle([[1,a],[2,b],[3,c]],What).
% rle(What,[a,b,b,c,c,c]).