support writeln/1 for SWI compatibility.

docs/yap.tex

@@ -4583,6 +4583,12 @@ exists.
 The term @var{T} is written to the current output stream according to
 the operator declarations in force.
 
+@item writeln(@var{T}) [ISO]
+@findex writeln/1
+@snindex writeln/1
+@cnindex writeln/1
+Same as @code{write/1} followed by {nl/0}.
+
 @item display(+@var{T})
 @findex display/1
 @syindex display/1

pl/yio.yap

@@ -506,6 +506,10 @@ nl.
 write(T) :-
 	'$write'(4, T).
 
+writeln(T) :-
+	'$write'(4, T),
+	nl.	
+
 write(Stream,T) :- 
 	'$write'(Stream,4,T).