Q3 Program
Given a structure, write a program \texttt{leaves/2}, to be used in the mode \texttt{leaves(+,-)}, to collect
the leaves of the structure into a list, thus:
\texttt{leaves(a, X)} X= [a]
\texttt{leaves (f(a,b,c), X)} X= [a,b,c]
\texttt{leaves (f(a,g(b,c), h(d,e,f)),X)} X=[a,b,c,d,e,f]
Hint you may use the built in predicate $=../2$ and the predicate \texttt{append/3}.