implement epsilon (from ISO)

docs/yap.tex

@@ -3776,9 +3776,12 @@ Arc cosine.
 @item atan(@var{X}) [ISO]
 Arc tangent.
 
-@item atan2(@var{X})
+@item atan(@var{X},@var{Y})
 Four-quadrant arc tangent.
 
+@item atan2(@var{X},@var{Y})
+Four-quadrant arc tangent, same as atan/2.
+
 @item sinh(@var{X})
 Hyperbolic sine.
 
@@ -3930,6 +3933,10 @@ diameter.
 @item e
 The base of the natural logarithms.
 
+@item epsilon
+The difference between the float @code{1.0} and the first larger floating point
+number.
+
 @item inf
 Infinity according to the IEEE Floating-Point standard. Note that
 evaluating this term will generate a domain error in the @code{iso}