Give the most general unifier (if it exists) of the following atoms (recall
that [1,2] is short for the list [1|[2|[]]]):
a) append([1,2],X,U), append([Y|L],Z,[Y|R])
b) append([1,2],X,[0,1]), append([Y|L],Z,[Y|R])
c) append([],X,[0,1]), append([Y|L],Z,[Y|R])
d) append([],X,[0]), append([],[X|L],[Y])