Merge epic/79-rnc-nameclass-subtraction: name-class subtraction

Part of #79

Sources/Schema/RelaxNGCompactParser.swift

@@ -308,6 +308,11 @@ private extension RNCParser {
         switch peek() {
         case .symbol("*"):
             advance()
+            // `* - nameClass` is any name except the subtracted class.
+            if peek() == .symbol("-") {
+                advance()
+                return .anyNameExcept(parseSimpleNameClass(attribute: attribute))
+            }
             return .anyName
         case .symbol("("):
             advance()

Tests/RelaxNGNameClassTests.swift

@@ -0,0 +1,22 @@
+@testable import PureXML
+import Testing
+
+@Suite("RELAX NG compact name-class subtraction")
+struct RelaxNGNameClassTests {
+    private func valid(_ rnc: String, _ xml: String) throws -> Bool {
+        try PureXML.Schema.RelaxNG(compact: rnc).validate(xml)
+    }
+
+    @Test("* - name admits any element except the subtracted one")
+    func test_subtraction() throws {
+        let rnc = "element root { element * - skip { empty }* }"
+        #expect(try valid(rnc, "<root><a/><b/></root>"))
+        #expect(try !valid(rnc, "<root><skip/></root>"))
+    }
+
+    @Test("A plain wildcard still admits everything")
+    func test_plainWildcard() throws {
+        let rnc = "element root { element * { empty }* }"
+        #expect(try valid(rnc, "<root><skip/><a/></root>"))
+    }
+}