Invalid Filter-Construction for a GetFeature-Request on WFS 1.0.0. The Class org.geotools.xml.filter.FilterOpsComplexTypes.FilterType creates two filter terms (Line 885 - 891) by calling encodeFilter() twice. The first time for the real filter, the second time for a Fid-Filter (usually empty). The result is a filter-element with a nested filter-element. This is not allowed by the OGC Filter Specification and causes errors on some implementing WFS.