diff --git a/C/eval.c b/C/eval.c
index 21634f13f..142f4dce4 100644
--- a/C/eval.c
+++ b/C/eval.c
@@ -254,6 +254,74 @@ p_isinf( USES_REGS1 )
   return isinf(FloatOfTerm(out));
 }
 
+static Int
+p_logsum( USES_REGS1 )
+{                               /* X is Y        */
+  Term t1 = Deref(ARG1);
+  Term t2 = Deref(ARG2);
+  int done = FALSE;
+  Float f1, f2;
+  
+  while (!done) {
+    if (IsFloatTerm(t1)) {
+      f1 = FloatOfTerm(t1);
+      done = TRUE;
+    } else if (IsIntegerTerm(t1)) {
+      f1 = IntegerOfTerm(t1);
+      done = TRUE;
+    } else if (IsBigIntTerm(t1)) {
+      f1 = Yap_gmp_to_float(t1);
+      done = TRUE;
+    } else {
+      while (!(t1 = Eval(t1 PASS_REGS))) {
+	if (LOCAL_Error_TYPE == RESOURCE_ERROR_STACK) {
+	  LOCAL_Error_TYPE = YAP_NO_ERROR;
+	  if (!Yap_gcl(LOCAL_Error_Size, 1, ENV, CP)) {
+	    Yap_Error(RESOURCE_ERROR_STACK, ARG2, LOCAL_ErrorMessage);
+	    return FALSE;
+	  }
+	} else {
+	  Yap_Error(LOCAL_Error_TYPE, LOCAL_Error_Term, LOCAL_ErrorMessage);
+	  return FALSE;
+	}
+      }
+    }
+  }
+  done = FALSE;
+  while (!done) {
+    if (IsFloatTerm(t2)) {
+      f2 = FloatOfTerm(t2);
+      done = TRUE;
+    } else if (IsIntegerTerm(t2)) {
+      f2 = IntegerOfTerm(t2);
+      done = TRUE;
+    } else if (IsBigIntTerm(t2)) {
+      f2 = Yap_gmp_to_float(t2);
+      done = TRUE;
+    } else {
+      while (!(t2 = Eval(t2 PASS_REGS))) {
+	if (LOCAL_Error_TYPE == RESOURCE_ERROR_STACK) {
+	  LOCAL_Error_TYPE = YAP_NO_ERROR;
+	  if (!Yap_gcl(LOCAL_Error_Size, 2, ENV, CP)) {
+	    Yap_Error(RESOURCE_ERROR_STACK, ARG2, LOCAL_ErrorMessage);
+	    return FALSE;
+	  }
+	} else {
+	  Yap_Error(LOCAL_Error_TYPE, LOCAL_Error_Term, LOCAL_ErrorMessage);
+	  return FALSE;
+	}
+      }
+    }
+  }
+  if (f1 >= f2) {
+    Float fi = exp(f2-f1);
+    return Yap_unify(ARG3,MkFloatTerm(f1+log(1+fi)));
+  } else {
+    Float fi = exp(f1-f2);
+    return Yap_unify(ARG3,MkFloatTerm(f2+log(1+fi)));
+  }
+}
+
 Int
 Yap_ArithError(yap_error_number type, Term where, char *format,...)
 {
@@ -418,6 +486,7 @@ Yap_InitEval(void)
   Yap_InitCPred("is", 2, p_is, 0L);
   Yap_InitCPred("isnan", 1, p_isnan, TestPredFlag);
   Yap_InitCPred("isinf", 1, p_isinf, TestPredFlag);
+  Yap_InitCPred("logsum", 3, p_logsum, TestPredFlag);
   Yap_InitCPredBack("between", 3, 2, init_between, cont_between, 0);
 }
 
diff --git a/docs/yap.tex b/docs/yap.tex
index 743b4270d..d9d4f73f4 100644
--- a/docs/yap.tex
+++ b/docs/yap.tex
@@ -4459,6 +4459,15 @@ The following predicates provide counting:
     True if @var{Int3} = @var{Int1} + @var{Int2}. At least two of the
     three arguments must be instantiated to integers.
 
+@item logsum(+@var{Log1}, +@var{Log2}, -@var{Out} )
+@findex logsum/3
+@snindex logsum/3
+@cnindex logsum/3
+ True if @var{Log1} is the logarithm of the positive number @var{A1},
+ @var{Log2} is the logarithm of the positive number @var{A2}, and
+ @var{Out} is the logarithm of the sum of the numbers @var{A1} and
+ @var{A2}. Useful in probability computation.
+
 @item isnan(+@var{Float})
 @findex isnan/1
 @snindex isnan/1