(Math Functions): Document the optional second arg of atan.

1 files changed, 6 insertions, 3 deletions

diff --git a/lispref/numbers.texi b/lispref/numbers.texi
index 9689d5cedfe..e13bd04117a 100644
--- a/lispref/numbers.texi
+++ b/lispref/numbers.texi
@@ -1046,8 +1046,8 @@ pi
 is out of range (outside [-1, 1]), then the result is a NaN.
 @end defun
 
-@defun atan arg
-The value of @code{(atan @var{arg})} is a number between
+@defun atan y &optional x
+The value of @code{(atan @var{y})} is a number between
 @ifnottex
 @minus{}pi/2
 @end ifnottex
@@ -1061,7 +1061,10 @@ pi/2
 @tex
 @math{\pi/2}
 @end tex
-(exclusive) whose tangent is @var{arg}.
+(exclusive) whose tangent is @var{y}.  If the optional second
+argument @var{x} is given, the value of @code{(atan y x)} is the
+angle in radians between the vector @code{[@var{x}, @var{y}]} and the
+@code{X} axis.
 @end defun
 
 @defun exp arg