Add metaschema-framework#283 function-arity() and incomplete tests

There appear to be some issues parsing, seemingly at the ANLTRv4 grammar
layer, with the following kinds of function expressions that are in the
spec for example test cases for this function in the XPath 3.1 spec.

`fn:function-arity(fn:substring#2)`
`let $initial := fn:substring(?, 1, 1) return fn:function-arity($initial) returns 1`

More investigation is needed and perhaps separate bugs must be filed.

core/src/main/java/gov/nist/secauto/metaschema/core/metapath/function/library/DefaultFunctionLibrary.java

@@ -91,6 +91,8 @@ public DefaultFunctionLibrary() { // NOPMD - intentional
     // P2: https://www.w3.org/TR/xpath-functions-31/#func-format-integer
     // P2: https://www.w3.org/TR/xpath-functions-31/#func-format-number
     // P2: https://www.w3.org/TR/xpath-functions-31/#func-format-time
+    // https://www.w3.org/TR/xpath-functions-31/#func-function-arity
+    registerFunction(FnFunctionArity.SIGNATURE);
     // P1: https://www.w3.org/TR/xpath-functions-31/#func-generate-id
     // https://www.w3.org/TR/xpath-functions-31/#func-has-children
     registerFunction(FnHasChildren.SIGNATURE_NO_ARG);

core/src/main/java/gov/nist/secauto/metaschema/core/metapath/function/library/FnFunctionArity.java

@@ -0,0 +1,65 @@
+/*
+ * SPDX-FileCopyrightText: none
+ * SPDX-License-Identifier: CC0-1.0
+ */
+
+package gov.nist.secauto.metaschema.core.metapath.function.library;
+
+import gov.nist.secauto.metaschema.core.metapath.DynamicContext;
+import gov.nist.secauto.metaschema.core.metapath.MetapathConstants;
+import gov.nist.secauto.metaschema.core.metapath.function.FunctionUtils;
+import gov.nist.secauto.metaschema.core.metapath.function.IArgument;
+import gov.nist.secauto.metaschema.core.metapath.function.IFunction;
+import gov.nist.secauto.metaschema.core.metapath.item.IItem;
+import gov.nist.secauto.metaschema.core.metapath.item.ISequence;
+import gov.nist.secauto.metaschema.core.metapath.item.atomic.IIntegerItem;
+import gov.nist.secauto.metaschema.core.util.ObjectUtils;
+
+import java.util.List;
+
+import edu.umd.cs.findbugs.annotations.NonNull;
+
+/**
+ * /** Implements <a href=
+ * "https://www.w3.org/TR/xpath-functions-31/#func-function-arity">fn:function-arity</a>
+ * functions.
+ */
+public final class FnFunctionArity {
+  @NonNull
+  private static final String NAME = "function-arity";
+  @NonNull
+  static final IFunction SIGNATURE = IFunction.builder()
+      .name(NAME)
+      .namespace(MetapathConstants.NS_METAPATH_FUNCTIONS)
+      .deterministic()
+      .contextDependent()
+      .focusIndependent()
+      .argument(IArgument.builder()
+          .name("func")
+          .type(IItem.type())
+          .one()
+          .build())
+      .returnType(IIntegerItem.type())
+      .returnOne()
+      .functionHandler(FnFunctionArity::execute)
+      .build();
+
+  @SuppressWarnings("unused")
+  @NonNull
+  private static ISequence<IIntegerItem> execute(@NonNull IFunction function,
+      @NonNull List<ISequence<?>> arguments,
+      @NonNull DynamicContext dynamicContext,
+      IItem focus) {
+    try {
+      IFunction fn = FunctionUtils.asType(ObjectUtils.requireNonNull(arguments.get(0).getFirstItem(true)));
+    	return ISequence.of(IIntegerItem.valueOf(fn.arity()));
+    } catch (Exception ex) {
+    	// TODO: Spec is unclear for no function provided
+		  return ISequence.empty();
+	  }
+  }
+
+  private FnFunctionArity() {
+    // disable construction
+  }
+}

core/src/test/java/gov/nist/secauto/metaschema/core/metapath/function/library/FnFunctionArityTest.java

@@ -0,0 +1,47 @@
+/*
+ * SPDX-FileCopyrightText: none
+ * SPDX-License-Identifier: CC0-1.0
+ */
+
+package gov.nist.secauto.metaschema.core.metapath.function.library;
+
+import static gov.nist.secauto.metaschema.core.metapath.TestUtils.integer;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import gov.nist.secauto.metaschema.core.metapath.ExpressionTestBase;
+import gov.nist.secauto.metaschema.core.metapath.IMetapathExpression;
+import gov.nist.secauto.metaschema.core.metapath.IMetapathExpression.ResultType;
+import gov.nist.secauto.metaschema.core.metapath.item.IItem;
+
+
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+import java.util.stream.Stream;
+
+import edu.umd.cs.findbugs.annotations.NonNull;
+
+class FnFunctionArityTest extends ExpressionTestBase {
+
+	private static Stream<Arguments> provideValues() { // NOPMD - false positive
+		return Stream.of(
+			// Arguments.of(
+			// 	integer(2),
+			// 	"fn:function-arity(fn:substring#2)"),
+			Arguments.of(
+				integer(1),
+				"function-arity(function($node){name($node)})")//,
+			// Arguments.of(
+			// 	integer(1),
+			// 	"let $initial := fn:substring(?, 1, 1) return fn:function-arity($initial)")
+		);
+	}
+
+	@ParameterizedTest
+	@MethodSource("provideValues")
+	void test(@NonNull IItem expected, @NonNull String metapath) {
+		assertEquals(expected,
+				IMetapathExpression.compile(metapath).evaluateAs(null, ResultType.ITEM, newDynamicContext()));
+	}
+}