update

readme.hints.md

@@ -412,4 +412,55 @@ This makes the module fully spec-compliant and suitable as the default open-sour
 7. **Fix validation**: Implement missing validation rules (EN16931, XRechnung CIUS)
 8. **Add PDF/A-3 compliance**: Implement proper PDF/A-3 compliance checking
 9. **Add digital signatures**: Support for digital signatures
-10. **Error recovery**: Implement proper error recovery for malformed XML
+10. **Error recovery**: Implement proper error recovery for malformed XML
+
+## Test Suite Compatibility Issue (2025-01-27)
+
+### Problem Identified
+Many test suites in the project are failing with "t.test is not a function" error. This is because:
+- Tests were written for tap.js v16+ which supports subtests via `t.test()`
+- Project uses @git.zone/tstest which only supports top-level `tap.test()`
+
+### Affected Test Suites
+- All parsing tests (test.parse-01 through test.parse-12)
+- All PDF operation tests (test.pdf-01 through test.pdf-12)
+- All performance tests (test.perf-01 through test.perf-12)
+- All security tests (test.sec-01 through test.sec-10)
+- All standards compliance tests (test.std-01 through test.std-10)
+- All validation tests (test.val-09 through test.val-14)
+
+### Root Cause
+The tests appear to have been written for a different testing framework or a newer version of tap that supports nested tests.
+
+### Solution Options
+1. **Refactor all tests**: Convert nested `t.test()` calls to separate `tap.test()` blocks
+2. **Upgrade testing framework**: Switch to a newer version of tap that supports subtests
+3. **Use a compatibility layer**: Create a wrapper that translates the test syntax
+
+### EN16931 Validation Implementation (2025-01-27)
+
+Successfully implemented EN16931 mandatory field validation to make the library more spec-compliant:
+
+1. **Created EN16931Validator class** in `ts/formats/validation/en16931.validator.ts`
+   - Validates mandatory fields according to EN16931 business rules
+   - Validates ISO 4217 currency codes
+   - Throws descriptive errors for missing/invalid fields
+
+2. **Integrated validation into decoders**:
+   - XRechnungDecoder
+   - FacturXDecoder
+   - ZUGFeRDDecoder
+   - ZUGFeRDV1Decoder
+   
+3. **Added validation to EInvoice.toXmlString()**
+   - Validates mandatory fields before encoding
+   - Ensures spec compliance for all exports
+
+4. **Fixed error-handling tests**:
+   - ERR-02: Validation errors test - Now properly throws on invalid XML
+   - ERR-05: Memory errors test - Now catches validation errors
+   - ERR-06: Concurrent errors test - Now catches validation errors
+   - ERR-10: Configuration errors test - Now validates currency codes
+
+### Results
+All error-handling tests are now passing. The library is more spec-compliant by enforcing EN16931 mandatory field requirements.