Explain what this prolog program does?

I am working on Prolog and I came across the below program. When I execute this program with a query for example: mysterious([2,3,4,5,6,7,8],L). It has given me the answer 20. I did not understand the flow of getting this result. can anyone help me to understand this code?...How does it produce the answer 20 for the above mentioned query?

mysterious([X,Y|Xs], Res) :-
    mysterious(Xs, Res1),
    Res is X + Res1.


Because this is such a basic program, I will give you a hint. First off, this:

foo([X,Y|Rest]) :-

could be also written as:

foo([First|Rest]) :-
    bar(Rest, First).

bar([], Last).
bar([This|Rest], Prev) :-

It is unnecessarily verbose, but more explicit. In your case, however, it makes something obvious:

mysterious([], 0).
mysterious([X|Rest], Result) :-
    myst_1(Rest, X, Result).

myst_1([], Result, Result).
myst_1([_Y|Rest], X, Result) :-
    mysterious(Rest, Result1),
    Result is X + Result1.

Something which you did not mention in your question is that the compiler must have given you a "singleton variable" warning on the Y in the last clause of mysterious/2. This should have been a strong indication of what is going on already.

I can help you by commenting lines, maybe you will understand the prolog a little more, let's start:

Prolog programs describe relations, defined by means of clauses. Pure Prolog is restricted to Horn clauses. There are two types of clauses: facts and rules. (by wikipedia)

% comment in prolog starts with '%'
mysterious([],0).  % its called 'predicate', something like functions in imperative
                   % languages. it has two arguments: [], and 0. and it is ended
                   % by dot (.). Clauses with empty bodies are called facts. 
                   % (this is fact).

mysterious([X],X). % is actually a "rule" that says, "if the list has one element, 
                   % then the result is just that element". (by @mbratch)

mysterious([X,Y|Xs], Res) :- % now, when argument 1 is list which looks like
                             % [SomeElement, NextElement|LastElements],
  mysterious(Xs, Res1),      % call 'mysterious' with only LastElements 
                             % and variable Res1 (skip first two elements of list)
  Res is X + Res1.           % then add first element to Res1.

% it calculates the sum of every second element of list, beginning on first.

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