(Scientific): Add two-argument atan.

diff --git a/doc/ref/scheme-data.texi b/doc/ref/scheme-data.texi
index 5046f05..724a8c4 100755 (executable)
--- a/doc/ref/scheme-data.texi
+++ b/doc/ref/scheme-data.texi
@@ -859,7 +859,8 @@ Return the arccosine of @var{z}.
 @rnindex atan
 @c begin (texi-doc-string "guile" "atan")
 @deffn {Scheme Procedure} atan z
-Return the arctangent of @var{z}.
+@deffnx {Scheme Procedure} atan y x
+Return the arctangent of @var{z}, or of @math{@var{y}/@var{x}}.
 @end deffn
 
 @rnindex exp