diff --git a/core/src/main/java/io/parsingdata/metal/expression/value/Expand.java b/core/src/main/java/io/parsingdata/metal/expression/value/Expand.java
index 1e9b5ebd..34fff92d 100644
--- a/core/src/main/java/io/parsingdata/metal/expression/value/Expand.java
+++ b/core/src/main/java/io/parsingdata/metal/expression/value/Expand.java
@@ -22,7 +22,6 @@
 import static io.parsingdata.metal.expression.value.NotAValue.NOT_A_VALUE;
 
 import java.util.Objects;
-import java.util.Optional;
 
 import io.parsingdata.metal.Trampoline;
 import io.parsingdata.metal.Util;
@@ -40,7 +39,7 @@
  * of the result of evaluating <code>bases</code> are concatenated. The amount
  * of copies equals the result of evaluating <code>count</code>. If
  * <code>count</code> evaluated to an empty value or <code>NOT_A_VALUE</code>,
- * an IllegalArgumentException is thrown.
+ * an {@link IllegalArgumentException} is thrown.
  */
 public class Expand implements ValueExpression {
 
@@ -58,11 +57,10 @@ public ImmutableList<Value> eval(final ParseState parseState, final Encoding enc
         if (baseList.isEmpty()) {
             return baseList;
         }
-        final Optional<Value> countValue = count.evalSingle(parseState, encoding);
-        if (!countValue.isPresent() || countValue.get().equals(NOT_A_VALUE)) {
-            throw new IllegalArgumentException("Count must evaluate to a non-empty countable value.");
-        }
-        return expand(baseList, countValue.get().asNumeric().intValueExact(), new ImmutableList<>()).computeResult();
+        return count.evalSingle(parseState, encoding)
+            .filter(countValue -> !countValue.equals(NOT_A_VALUE))
+            .map(countValue -> expand(baseList, countValue.asNumeric().intValueExact(), new ImmutableList<>()).computeResult())
+            .orElseThrow(() -> new IllegalArgumentException("Count must evaluate to a non-empty countable value."));
     }
 
     private Trampoline<ImmutableList<Value>> expand(final ImmutableList<Value> baseList, final int countValue, final ImmutableList<Value> aggregate) {
diff --git a/core/src/main/java/io/parsingdata/metal/expression/value/SingleValueExpression.java b/core/src/main/java/io/parsingdata/metal/expression/value/SingleValueExpression.java
index 0bdfc5bd..51f67681 100644
--- a/core/src/main/java/io/parsingdata/metal/expression/value/SingleValueExpression.java
+++ b/core/src/main/java/io/parsingdata/metal/expression/value/SingleValueExpression.java
@@ -26,7 +26,7 @@
  * Interface for all SingleValueExpression implementations.
  * <p>
  * A SingleValueExpression is an expression that is evaluated by executing its
- * {@link #eval(ParseState, Encoding)} method. It yields an {@link Optional}
+ * {@link #evalSingle(ParseState, Encoding)} method. It yields an {@link Optional}
  * {@link Value} object.
  * <p>
  * As context, it receives the current <code>ParseState</code> object as
diff --git a/core/src/main/java/io/parsingdata/metal/expression/value/reference/Nth.java b/core/src/main/java/io/parsingdata/metal/expression/value/reference/Nth.java
index e9f5ff41..51679a52 100644
--- a/core/src/main/java/io/parsingdata/metal/expression/value/reference/Nth.java
+++ b/core/src/main/java/io/parsingdata/metal/expression/value/reference/Nth.java
@@ -33,6 +33,7 @@
 import io.parsingdata.metal.data.ImmutableList;
 import io.parsingdata.metal.data.ParseState;
 import io.parsingdata.metal.encoding.Encoding;
+import io.parsingdata.metal.expression.value.NotAValue;
 import io.parsingdata.metal.expression.value.Value;
 import io.parsingdata.metal.expression.value.ValueExpression;
 
@@ -43,8 +44,8 @@
  * <code>indices</code> (both {@link ValueExpression}s). Both operands are
  * evaluated. Next, the resulting values of evaluating <code>indices</code> is
  * used as a list of integer indices into the results of evaluating
- * <code>values</code>. For every invalid index (<code>NOT_A_VALUE</code>, a
- * negative value or an index that is out of bounds) <code>NOT_A_VALUE</code>
+ * <code>values</code>. For every invalid index ({@link NotAValue#NOT_A_VALUE}, a
+ * negative value or an index that is out of bounds) {@link NotAValue#NOT_A_VALUE}
  * is returned.
  */
 public class Nth implements ValueExpression {
diff --git a/core/src/main/java/io/parsingdata/metal/expression/value/reference/Self.java b/core/src/main/java/io/parsingdata/metal/expression/value/reference/Self.java
index 8d5daca9..b4f5c392 100644
--- a/core/src/main/java/io/parsingdata/metal/expression/value/reference/Self.java
+++ b/core/src/main/java/io/parsingdata/metal/expression/value/reference/Self.java
@@ -16,6 +16,8 @@
 
 package io.parsingdata.metal.expression.value.reference;
 
+import static java.util.function.Function.identity;
+
 import java.util.Optional;
 
 import io.parsingdata.metal.Util;
@@ -32,7 +34,7 @@ public class Self implements SingleValueExpression {
 
     @Override
     public Optional<Value> evalSingle(final ParseState parseState, final Encoding encoding) {
-        return parseState.order.current().flatMap(Optional::of);
+        return parseState.order.current().map(identity());
     }
 
     @Override
diff --git a/core/src/main/java/io/parsingdata/metal/token/RepN.java b/core/src/main/java/io/parsingdata/metal/token/RepN.java
index 65ec83b9..482bfd63 100644
--- a/core/src/main/java/io/parsingdata/metal/token/RepN.java
+++ b/core/src/main/java/io/parsingdata/metal/token/RepN.java
@@ -55,7 +55,7 @@ protected Optional<ParseState> parseImpl(final Environment environment) {
             .filter(count -> !count.equals(NOT_A_VALUE))
             .flatMap(count -> parse(environment, env -> env.parseState.iterations.head.right.compareTo(count.asNumeric()) >= 0, env -> failure()));
     }
-    
+
     @Override
     public String toString() {
         return getClass().getSimpleName() + "(" + makeNameFragment() + token + "," + n + ")";