Steps to Reproduce

Override an unimplemented function using:
<?xel-method prefix="zss" name="EDATE"
class="package.package.CustomFunctions"
signature="org.zkoss.poi.ss.formula.eval.ValueEval myEdate(org.zkoss.poi.ss.formula.eval.ValueEval[], int, int)"?>
<zk>

Actual Result

When using the function in a cell, the result is: #NAME?

Expected Result

The cell should display the result of the custom function

Debug Info

Functions declared as unimplemented in org.zkoss.zssex.formula.ZKUDFFinder cannot be overriden.
https://github.com/zkoss/zsscml/blob/3.9.1/zssex/src/org/zkoss/zssex/formula/ZKUDFFinder.java#L47

Root Cause

Unimplemented function are suppressed by ZKUDFFinder

Workaround

replace the ZKUDFFinder class, and comment out the line(s) relevant to the function to override.