Narrow conditions for using Parse method

Closes #15

RegExtract.Test/Usage.cs

@@ -220,6 +220,25 @@ public void a012()
             Assert.Equivalent(("sxc", new List<Rule> { new Conditional(new Condition('x', '>', 2414), new Workflow("jtp")), new Conditional(new Condition('s', '>', 954), new Reject()), new Conditional(new Condition('m', '>', 2406), new Accept()), new Absolute(new Workflow("xfz")) }), result);
         }
 
+        record triple(int a, int b, int c) {
+            public static triple Parse(string str)
+            {
+                return new triple(0,0,0);
+            }
+        }
+
+        private Regex rxa013 = new(@"(\d+) (\d+) (\d+)");
+
+        [Fact]
+        public void a013()
+        {
+            var plan = CreateAndLogPlan<triple>(rxa013);
+
+            var result = plan.Extract("123 456 789");
+
+            Assert.Equal(new triple(123,456,789), result);
+        }
+
         [Fact]
         public void f001()
         {

RegExtract/ExtractionPlan.cs

@@ -132,11 +132,11 @@ private ExtractionPlanNode AssignTypesToTree(RegexCaptureGroupNode tree, Extract
             List<ExtractionPlanNode> groups = new();
             List<ExtractionPlanNode> namedgroups = new();
 
-            if (type.IsDirectlyConstructable)
+            if (tree.children is [] or [{children: []}] && type.IsDirectlyConstructable)
             {
                 // We're at a leaf in the type hierarchy, and all we need is a string.
                 // If there's an inner capture group, use it to narrow the match.
-                if (tree.children.Count() == 1)
+                if (tree.children.Length == 1)
                 {
                     tree = tree.children.Single();
                 }