diff --git a/src/Firely.Fhir.Validation/PublicAPI.Unshipped.txt b/src/Firely.Fhir.Validation/PublicAPI.Unshipped.txt
index 74c26f28..a4813d7e 100644
--- a/src/Firely.Fhir.Validation/PublicAPI.Unshipped.txt
+++ b/src/Firely.Fhir.Validation/PublicAPI.Unshipped.txt
@@ -584,3 +584,34 @@ virtual Firely.Fhir.Validation.InvariantValidator.PrintMembers(System.Text.Strin
 Firely.Fhir.Validation.NamedExtensionMapper
 Firely.Fhir.Validation.ValidationSettings.MapNamedElement -> Firely.Fhir.Validation.NamedExtensionMapper!
 virtual Firely.Fhir.Validation.NamedExtensionMapper.Invoke(string! jsonName) -> string?
+~override Firely.Fhir.Validation.AssertionStep.Equals(object obj) -> bool
+Firely.Fhir.Validation.AssertionContainerExtensions
+Firely.Fhir.Validation.AssertionStep
+Firely.Fhir.Validation.AssertionStep.Anchor.get -> string?
+Firely.Fhir.Validation.AssertionStep.AssertionStep() -> void
+Firely.Fhir.Validation.AssertionStep.ElementName.get -> string?
+Firely.Fhir.Validation.AssertionStep.Equals(Firely.Fhir.Validation.AssertionStep other) -> bool
+Firely.Fhir.Validation.AssertionStep.Kind.get -> Firely.Fhir.Validation.AssertionStepKind
+Firely.Fhir.Validation.AssertionStep.Name.get -> string?
+Firely.Fhir.Validation.AssertionStep.SliceName.get -> string?
+Firely.Fhir.Validation.AssertionStep.TargetType.get -> string?
+Firely.Fhir.Validation.AssertionStepKind
+Firely.Fhir.Validation.AssertionStepKind.Child = 1 -> Firely.Fhir.Validation.AssertionStepKind
+Firely.Fhir.Validation.AssertionStepKind.Member = 0 -> Firely.Fhir.Validation.AssertionStepKind
+Firely.Fhir.Validation.AssertionStepKind.ReferenceTarget = 4 -> Firely.Fhir.Validation.AssertionStepKind
+Firely.Fhir.Validation.AssertionStepKind.Slice = 2 -> Firely.Fhir.Validation.AssertionStepKind
+Firely.Fhir.Validation.AssertionStepKind.Subschema = 3 -> Firely.Fhir.Validation.AssertionStepKind
+Firely.Fhir.Validation.IAssertionContainer
+Firely.Fhir.Validation.IAssertionContainer.WithChildren(System.Func<Firely.Fhir.Validation.AssertionStep, Firely.Fhir.Validation.IAssertion!, Firely.Fhir.Validation.IAssertion!>! rewrite) -> Firely.Fhir.Validation.IAssertion!
+override Firely.Fhir.Validation.AssertionStep.GetHashCode() -> int
+override Firely.Fhir.Validation.AssertionStep.ToString() -> string!
+static Firely.Fhir.Validation.AssertionContainerExtensions.Children(this Firely.Fhir.Validation.IAssertionContainer! container) -> System.Collections.Generic.IReadOnlyList<(Firely.Fhir.Validation.AssertionStep Step, Firely.Fhir.Validation.IAssertion! Child)>!
+static Firely.Fhir.Validation.AssertionContainerExtensions.TryRewriteItems<T>(this System.Collections.Generic.IReadOnlyCollection<T>! items, System.Func<T, (Firely.Fhir.Validation.AssertionStep Step, Firely.Fhir.Validation.IAssertion! Child)>! select, System.Func<T, Firely.Fhir.Validation.IAssertion!, T>! replace, System.Func<Firely.Fhir.Validation.AssertionStep, Firely.Fhir.Validation.IAssertion!, Firely.Fhir.Validation.IAssertion!>! rewrite) -> T[]?
+static Firely.Fhir.Validation.AssertionContainerExtensions.TryRewriteMembers(this System.Collections.Generic.IReadOnlyCollection<Firely.Fhir.Validation.IAssertion!>! members, Firely.Fhir.Validation.AssertionStep step, System.Func<Firely.Fhir.Validation.AssertionStep, Firely.Fhir.Validation.IAssertion!, Firely.Fhir.Validation.IAssertion!>! rewrite) -> Firely.Fhir.Validation.IAssertion![]?
+static Firely.Fhir.Validation.AssertionStep.Child(string! elementName) -> Firely.Fhir.Validation.AssertionStep
+static Firely.Fhir.Validation.AssertionStep.Member() -> Firely.Fhir.Validation.AssertionStep
+static Firely.Fhir.Validation.AssertionStep.operator !=(Firely.Fhir.Validation.AssertionStep left, Firely.Fhir.Validation.AssertionStep right) -> bool
+static Firely.Fhir.Validation.AssertionStep.operator ==(Firely.Fhir.Validation.AssertionStep left, Firely.Fhir.Validation.AssertionStep right) -> bool
+static Firely.Fhir.Validation.AssertionStep.ReferenceTarget(string? targetType) -> Firely.Fhir.Validation.AssertionStep
+static Firely.Fhir.Validation.AssertionStep.Slice(string! sliceName) -> Firely.Fhir.Validation.AssertionStep
+static Firely.Fhir.Validation.AssertionStep.Subschema(string! anchor) -> Firely.Fhir.Validation.AssertionStep
diff --git a/src/Firely.Fhir.Validation/Schema/IAssertionContainer.cs b/src/Firely.Fhir.Validation/Schema/IAssertionContainer.cs
index 71ca2de3..c9aca7c8 100644
--- a/src/Firely.Fhir.Validation/Schema/IAssertionContainer.cs
+++ b/src/Firely.Fhir.Validation/Schema/IAssertionContainer.cs
@@ -8,6 +8,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.ComponentModel;
 
 namespace Firely.Fhir.Validation
 {
@@ -18,7 +19,13 @@ namespace Firely.Fhir.Validation
     /// <see cref="PathStack"/> during validation. Only steps that are visible in the compiled schema
     /// exist here: the instance-side events (indexes, internal references, resource starts) have no
     /// meaning while rewriting a schema.</remarks>
-    internal enum AssertionStepKind
+    [EditorBrowsable(EditorBrowsableState.Never)]
+#if NET8_0_OR_GREATER
+    [System.Diagnostics.CodeAnalysis.Experimental(diagnosticId: "ExperimentalApi")]
+#else
+    [System.Obsolete("This function is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.")]
+#endif
+    public enum AssertionStepKind
     {
         /// <summary>
         /// A member of the container that does not change the position within the instance: a member of an
@@ -62,8 +69,35 @@ internal enum AssertionStepKind
     /// </summary>
     /// <remarks>The step describes how the nested assertion is reached, not what it is: identity (canonical,
     /// version, schema id) is carried by the schemas encountered along the way, not by the steps.</remarks>
-    internal readonly record struct AssertionStep(AssertionStepKind Kind, string? Name = null)
+    [EditorBrowsable(EditorBrowsableState.Never)]
+#if NET8_0_OR_GREATER
+    [System.Diagnostics.CodeAnalysis.Experimental(diagnosticId: "ExperimentalApi")]
+#else
+    [System.Obsolete("This function is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.")]
+#endif
+    public readonly record struct AssertionStep
     {
+        // Steps are only created by the factory methods below, so every step has a name exactly when its
+        // kind requires one. Note that default(AssertionStep) is a Member step, which is valid as well.
+        private AssertionStep(AssertionStepKind kind, string? name = null)
+        {
+            Kind = kind;
+            Name = name;
+        }
+
+        /// <summary>
+        /// The kind of step.
+        /// </summary>
+        public AssertionStepKind Kind { get; }
+
+        /// <summary>
+        /// The name that goes with the kind of step: the element name, slice name, anchor or target type.
+        /// Always <c>null</c> for a <see cref="AssertionStepKind.Member"/> step, and never <c>null</c> for a
+        /// <see cref="AssertionStepKind.Child"/>, <see cref="AssertionStepKind.Slice"/> or
+        /// <see cref="AssertionStepKind.Subschema"/> step.
+        /// </summary>
+        public string? Name { get; }
+
         /// <summary>
         /// A step to a member that does not change the position within the instance.
         /// </summary>
@@ -72,17 +106,20 @@ internal readonly record struct AssertionStep(AssertionStepKind Kind, string? Na
         /// <summary>
         /// A step to the assertions for the child element with the given name.
         /// </summary>
-        public static AssertionStep Child(string elementName) => new(AssertionStepKind.Child, elementName);
+        public static AssertionStep Child(string elementName) =>
+            new(AssertionStepKind.Child, elementName ?? throw new ArgumentNullException(nameof(elementName)));
 
         /// <summary>
         /// A step to the assertions for the slice with the given name.
         /// </summary>
-        public static AssertionStep Slice(string sliceName) => new(AssertionStepKind.Slice, sliceName);
+        public static AssertionStep Slice(string sliceName) =>
+            new(AssertionStepKind.Slice, sliceName ?? throw new ArgumentNullException(nameof(sliceName)));
 
         /// <summary>
         /// A step to the subschema with the given anchor.
         /// </summary>
-        public static AssertionStep Subschema(string anchor) => new(AssertionStepKind.Subschema, anchor);
+        public static AssertionStep Subschema(string anchor) =>
+            new(AssertionStepKind.Subschema, anchor ?? throw new ArgumentNullException(nameof(anchor)));
 
         /// <summary>
         /// A step to the schema for the targets of a reference of the given type, or for the targets of
@@ -130,10 +167,18 @@ internal readonly record struct AssertionStep(AssertionStepKind Kind, string? Na
     /// walked and rewritten without knowing the concrete container types.
     /// </summary>
     /// <remarks>Every <see cref="IAssertion"/> that keeps nested assertions as part of its state must
-    /// implement this interface - <c>AssertionContainerTests</c> enforces that for this assembly. Implement
-    /// it explicitly: it is an implementation detail of schema rewriting, not part of the assertion's
-    /// public surface.</remarks>
-    internal interface IAssertionContainer
+    /// implement this interface - <c>AssertionContainerTests</c> enforces that for this assembly. That
+    /// includes custom building blocks defined outside this library: an assertion that holds nested
+    /// assertions but does not implement this interface is opaque to schema rewriters, which then silently
+    /// leave everything nested in it unchanged. Implement it explicitly: it is an implementation detail of
+    /// schema rewriting, not part of the assertion's public surface.</remarks>
+    [EditorBrowsable(EditorBrowsableState.Never)]
+#if NET8_0_OR_GREATER
+    [System.Diagnostics.CodeAnalysis.Experimental(diagnosticId: "ExperimentalApi")]
+#else
+    [System.Obsolete("This function is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.")]
+#endif
+    public interface IAssertionContainer
     {
         /// <summary>
         /// Returns a copy of this container in which every nested assertion is replaced by the result of
@@ -155,7 +200,17 @@ internal interface IAssertionContainer
         IAssertion WithChildren(Func<AssertionStep, IAssertion, IAssertion> rewrite);
     }
 
-    internal static class AssertionContainerExtensions
+    /// <summary>
+    /// Helpers for walking <see cref="IAssertionContainer"/>s, and for implementing
+    /// <see cref="IAssertionContainer.WithChildren(Func{AssertionStep, IAssertion, IAssertion})"/>.
+    /// </summary>
+    [EditorBrowsable(EditorBrowsableState.Never)]
+#if NET8_0_OR_GREATER
+    [System.Diagnostics.CodeAnalysis.Experimental(diagnosticId: "ExperimentalApi")]
+#else
+    [System.Obsolete("This function is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.")]
+#endif
+    public static class AssertionContainerExtensions
     {
         /// <summary>
         /// Enumerates the nested assertions of a container, with the step leading to each of them.
diff --git a/test/Firely.Fhir.Validation.Tests/Schema/AssertionContainerTests.cs b/test/Firely.Fhir.Validation.Tests/Schema/AssertionContainerTests.cs
index 0107e876..7caa3a0a 100644
--- a/test/Firely.Fhir.Validation.Tests/Schema/AssertionContainerTests.cs
+++ b/test/Firely.Fhir.Validation.Tests/Schema/AssertionContainerTests.cs
@@ -215,6 +215,17 @@ public void ReferenceTargetsCarryTheirType()
             Assert.AreEqual("->*", untypedStep.ToString());
         }
 
+        [TestMethod]
+        public void NamedStepsRequireTheirName()
+        {
+            Assert.ThrowsException<ArgumentNullException>(() => AssertionStep.Child(null!));
+            Assert.ThrowsException<ArgumentNullException>(() => AssertionStep.Slice(null!));
+            Assert.ThrowsException<ArgumentNullException>(() => AssertionStep.Subschema(null!));
+
+            // the default step is a (nameless) member, which is a valid step
+            Assert.AreEqual(AssertionStep.Member(), default(AssertionStep));
+        }
+
         #endregion
 
         #region an unchanged container keeps its identity