7.12.2
Extending filter using the fes:Function element
Filter may be extended by adding new functions to the syntax that are invoked using the fes:Function element (see 7.6). Any functions added to the filter syntax shall be listed in the filter capabilities document using the fes:Function element (see 7.14.6). It is strongly recommended that additional functions be fully documented in the filter capabilities document using comments or the ows:Metadata element (see 7.14.6).
7.12.3
Extending filter by adding new elements
The filter syntax may also be extended by adding new elements that represent new operators. This may be accomplished by adding new elements that are substitutable for one of fes:comparisonOps, fes:spatialOps, fes:temporalOps, or fes:extensionOps (see 7.2).
Any new operators that are added to the filter syntax shall be defined in a namespace other than the filter namespace (see 5.4).
Any new operators that are substitutable for fes:comparisonOps, fes:spatialOps or fes:temporalOps shall be listed in the capabilities document using the pattern “extension:Name_of_new_Operator” (see 7.7, 7.8, 7.9) where the token “Name_of_new_Operator” is a placeholder for the name of the new operator.
Any new operators that are substitutable for fes:extensionOps shall be listed in the fes:Extended_Capabilities section of the filter capabilities document (see 7.14.7).
EXAMPLE The following XML Schema illustrates how to add a new comparison operator named “myops:PropertyExists” to the filter syntax.