Lost value.

Hi everyone.

I use SWI-Prolog version 5.6.27.

Here is my code :
go :-
write('y or n : '),
read(X),

% Here I get what I wrote
writeln(X),
X == y, writeln(yes);

% Here, the value is lost
writeln(X), writeln(no).

Why do I obtain this ?

9 ?- go.
y or n : y.
before the test : y
yes

Yes
10 ?- go.
y or n : n.
before the test : n
after the test : _L129
no

When the test fails, I loose the value !

Can you explain please.

Thank you.
Yes

.