diff --git a/C/arith0.c b/C/arith0.c
index 289447962..2e68c2375 100644
--- a/C/arith0.c
+++ b/C/arith0.c
@@ -49,6 +49,11 @@ static char     SccsId[] = "%W% %G%";
 #define NAN      (0.0/0.0)
 #endif
 
+/* copied from SWI-Prolog */
+#ifndef DBL_EPSILON /* normal for IEEE 64-bit double */
+#define DBL_EPSILON 0.00000000000000022204
+#endif
+
 static Term
 eval0(Int fi) {
   arith0_op fop = fi;
@@ -61,6 +66,10 @@ eval0(Int fi) {
     {
       RFLOAT(M_E);
     }
+  case op_epsilon:
+    {
+      RFLOAT(DBL_EPSILON);
+    }
   case op_inf:
     {
 #ifdef _MSC_VER /* Microsoft's Visual C++ Compiler */
@@ -152,6 +161,7 @@ typedef struct init_const_eval {
 static InitConstEntry InitConstTab[] = {
   {"pi", op_pi},
   {"e", op_e},
+  {"epsilon", op_epsilon},
   {"inf", op_inf},
   {"nan", op_nan},
   {"random", op_random},
diff --git a/C/arith2.c b/C/arith2.c
index 2e99a3b21..bbeb381d9 100644
--- a/C/arith2.c
+++ b/C/arith2.c
@@ -1093,6 +1093,7 @@ static InitBinEntry InitBinTab[] = {
   {"\\/", op_or},
   {"#", op_xor},
   {"><", op_xor},
+  {"atan", op_atan2},
   {"atan2", op_atan2},
   /* C-Prolog exponentiation */
   {"^", op_power},
diff --git a/H/eval.h b/H/eval.h
index 7c87e06e2..e4e27e202 100644
--- a/H/eval.h
+++ b/H/eval.h
@@ -21,6 +21,9 @@
 #if HAVE_MATH_H
 #include <math.h>
 #endif
+#ifdef HAVE_FLOAT_H
+#include <float.h>
+#endif
 #ifdef HAVE_IEEEFP_H
 #include <ieeefp.h>
 #endif
@@ -42,6 +45,7 @@
 typedef enum {
   op_pi,
   op_e,
+  op_epsilon,
   op_inf,
   op_nan,
   op_random,
diff --git a/docs/yap.tex b/docs/yap.tex
index 3ad036e35..fd967d673 100644
--- a/docs/yap.tex
+++ b/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}