string_lower producing atoms

Since string_lower is documented in the section 5.2 of the manual which
deals with strings (in the SWI-Prolog 7 sense), and its description doesn't
mention atoms, I'd expect its output to be a string, but instead what I get
is this:

?- current_prolog_flag(double_quotes,X).
X = string.

?- string_lower("Foo", X), X = "foo".
false.

?- string_lower("Foo", X).
X = foo.

?- string_lower("Foo", "foo").
true.


Is there any particular reason for this? Isn't the point of strings to
avoid the overhead associated with atoms?

-- 
Peter Berry <[email protected]>
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