Lab 4

1. Discuss the solution to Assignment 1
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2. List operations
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1. List Manipulation

a. A predicate to compute the third element of a list 
(fails if the list has less elements).

?- third([a,b,c,d],X).
X = c 
yes

?- third([a,b],X).
No

b. Remove the first occurrence of an element from a list
?- rm-dups(a, [a,b,a,c,c],L).
L= [b,a,c,c]
yes

 Remove all occurrences of an element from a list
?- rm-dups(a, [a,b,a,a,c,c],L).
L= [b,c,c]
yes


c. Remove duplicate occurrences of an element

   rm-dups(L1,L2) -- L2 is the same as L1 with the 
second and succeeding immediately adjacent duplicates removed.
   For example:

?- rm-dups([a,b,a,a,a,c,c],[a,b,a,c]).
Yes
?- rm-dups([a,b,a,a,a,c,c],X).
X = [a, b, a, c] ;
No
?- rm-dups([a,b,a,a,a,c,c],[a,b,c]).
No


d. Count element occurrences.

    count(L,E,N) holds if the list L contains E N times.
    Precondition: L and E are instantiated. L is a non-nested list.

?- count([1,2,1,1],1,3).
Yes
?- count([1,2,1,1],1,N).
N = 3 ;
No
?- count([1,2,1,1],1,2).
No
?- count([1,2,[1,2],1],[1,2],N).
N = 1 ;
No