问使用saxon s9api的扩展函数

EN

Caused by: net.sf.saxon.trans.XPathException: Unknown system function follow()
    at net.sf.saxon.expr.parser.XPathParser.grumble(XPathParser.java:282)

<dependency>
   <groupId>net.sf.saxon</groupId>
   <artifactId>Saxon-HE</artifactId>
   <version>9.7.0-14</version>
</dependency>

import net.sf.saxon.expr.XPathContext;
import net.sf.saxon.lib.ExtensionFunctionCall;
import net.sf.saxon.lib.ExtensionFunctionDefinition;
import net.sf.saxon.om.Sequence;
import net.sf.saxon.om.StructuredQName;
import net.sf.saxon.s9api.Processor;
import net.sf.saxon.s9api.XPathCompiler;
import net.sf.saxon.s9api.XPathExecutable;
import net.sf.saxon.trans.XPathException;
import net.sf.saxon.value.SequenceType;

public class FollowTest {
    public static void main(String[] args) throws Exception {
        new FollowTest().test();
    }

    private void test () throws Exception {
        Processor proc = new Processor(false);
        proc.registerExtensionFunction(new Follow());
        XPathCompiler xx = proc.newXPathCompiler();
        XPathExecutable x = xx.compile("follow(/a/b/c)/type='xyz'");
    }

    public class Follow extends ExtensionFunctionDefinition {

        @Override
        public StructuredQName getFunctionQName() {
            return new StructuredQName("", "http://example.com/saxon-extension", "follow");
        }

        @Override
        public int getMinimumNumberOfArguments() {
            return 1;
        }

        @Override
        public int getMaximumNumberOfArguments() {
            return 1;
        }

        @Override
        public SequenceType[] getArgumentTypes() {
            return new net.sf.saxon.value.SequenceType[] {SequenceType.SINGLE_STRING,};
        }

        @Override
        public SequenceType getResultType(SequenceType[] suppliedArgumentTypes) {
            return SequenceType.NODE_SEQUENCE;
        }

        @Override
        public boolean trustResultType() {
            return true;
        }

        @Override
        public boolean dependsOnFocus() {
            return false;
        }

        @Override
        public boolean hasSideEffects() {
            return false;
        }

        @Override
        public ExtensionFunctionCall makeCallExpression() {
            return null;
        }
        private class followCall extends ExtensionFunctionCall {
            @Override
            public Sequence call(XPathContext context, Sequence[] arguments) throws XPathException {
                return null;
            }
        }
    }
}