diff --git a/src/utils/firstInversion.js b/src/utils/firstInversion.js
new file mode 100644
index 0000000..2b68a12
--- /dev/null
+++ b/src/utils/firstInversion.js
@@ -0,0 +1,23 @@
+
+/**
+ * Returns k <= right such that [left,k[ is sorted. If k < right, then
+ * compare( array[k-1] , array[k] ) > 0.
+ */
+
+export function firstInversion ( compare , array , left , right ) {
+
+	if ( left >= right ) return right ;
+
+	while ( ++left < right ) {
+
+		if ( compare( array[left-1] , array[left] ) > 0 ) {
+
+			break ;
+
+		}
+
+	}
+
+	return left ;
+
+}
diff --git a/src/utils/index.js b/src/utils/index.js
index 5265a41..3cf77a8 100644
--- a/src/utils/index.js
+++ b/src/utils/index.js
@@ -1,2 +1,3 @@
-export * from './issorted' ;
+export * from './firstInversion' ;
+export * from './isSorted' ;
 export * from './whole' ;
diff --git a/src/utils/isSorted.js b/src/utils/isSorted.js
new file mode 100644
index 0000000..fc817a9
--- /dev/null
+++ b/src/utils/isSorted.js
@@ -0,0 +1,8 @@
+import {firstInversion} from './firstInversion';
+
+/**
+ * Returns whether range [left,right[ of array is sorted.
+ */
+export function isSorted ( compare , array , left , right ) {
+	return firstInversion(compare, array, left, right) === right;
+}
diff --git a/src/utils/issorted.js b/src/utils/issorted.js
deleted file mode 100644
index 3b15780..0000000
--- a/src/utils/issorted.js
+++ /dev/null
@@ -1,23 +0,0 @@
-
-/**
- * Checks whether range [left,right[ of array is sorted. Returns k <= right
- * such that [left,k[ is sorted.
- */
-
-export function issorted ( compare , array , left , right ) {
-
-	if ( left >= right ) return right ;
-
-	while ( ++left < right ) {
-
-		if ( compare( array[left-1] , array[left] ) > 0 ) {
-
-			break ;
-
-		}
-
-	}
-
-	return left ;
-
-}
diff --git a/test/src/issorted.js b/test/src/firstInversion.js
similarity index 74%
rename from test/src/issorted.js
rename to test/src/firstInversion.js
index 01f592f..145aae5 100644
--- a/test/src/issorted.js
+++ b/test/src/firstInversion.js
@@ -2,16 +2,16 @@ import test from 'ava' ;
 
 import { increasing , decreasing } from "@aureooms/js-compare" ;
 
-import { issorted } from '../../src' ;
+import { firstInversion } from '../../src' ;
 
 function macro ( t , array , left , right , k1 , k2 ) {
 
 	const n = array.length ;
 
-	t.is( issorted( increasing , array , left , right ) , k1 ) ;
-	t.is( issorted( decreasing , array , left , right ) , k2 ) ;
+	t.is( k1, firstInversion( increasing , array , left , right ) ) ;
+	t.is( k2, firstInversion( decreasing , array , left , right ) ) ;
 
-	t.is( array.length , n ) ;
+	t.is( n, array.length ) ;
 
 }
 
diff --git a/test/src/isSorted.js b/test/src/isSorted.js
new file mode 100644
index 0000000..1706559
--- /dev/null
+++ b/test/src/isSorted.js
@@ -0,0 +1,26 @@
+import test from 'ava' ;
+
+import { increasing , decreasing } from "@aureooms/js-compare" ;
+
+import { isSorted } from '../../src' ;
+
+function macro ( t , array , left , right , k1 , k2 ) {
+
+	const n = array.length ;
+
+	t.is( k1, isSorted( increasing , array , left , right ) ) ;
+	t.is( k2, isSorted( decreasing , array , left , right ) ) ;
+
+	t.is( n, array.length ) ;
+
+}
+
+macro.title = ( _ , ...args ) => args.join(' , ') ;
+
+test( macro , [ ] , 0 , 0 , true , true ) ;
+test( macro , [ 0 , 1 , 2 ] , 1 , 1 , true , true ) ;
+test( macro , [ 1 , 1 , 1 ] , 0 , 3 , true , true ) ;
+test( macro , [ 1 , 2 , 3 ] , 0 , 3 , true , false ) ;
+test( macro , [ 1 , 2 , 4 , 3 ] , 0 , 4 , false , false ) ;
+test( macro , [ 1 , 0 , 1 , 1 , 2 , 3 , 1 , 0 , 1 ] , 3 , 6 , true , false ) ;
+test( macro , [ 1 , 0 , 1 , 1 , 2 , 3 , 1 , 0 , 1 ] , 0 , 9 , false , false ) ;
diff --git a/test/src/whole.js b/test/src/whole.js
index fda6953..dac17ca 100644
--- a/test/src/whole.js
+++ b/test/src/whole.js
@@ -25,8 +25,8 @@ function check ( sortname, arraysort, ctor, n, comparename, compare ) {
 		shuffle( a, 0, n );
 		arraysort( compare, a );
 
-		t.is( sort.issorted( compare , a , 0 , n ) , n , "check sorted" );
-		t.is( a.length, n, "check length a" );
+		t.true( sort.isSorted( compare , a , 0 , n ) , "check sorted" );
+		t.is( n, a.length, "check length a" );
 
 	} );
 }