help asked for Calc jump function with empty arguments

Hi,

I am working on bug 88576, regarding IFERROR and IFNA not giving expected results when not all 
mandatory arguments are given.
On checking their behaviour in various cases, I discovered that IFERROR, IFNA and IF do not handle 
empty arguments as they should be.
With empty argument I mean e.g. -IFERROR(something;) of IF(statement;;argument).
With IFERROR and IFNA these situation should return an empty value to be compatible with Excel.
With IF, the behaviour is described in ODFF1.2.

My question is, where should this behaviour best be put in the code?
In class FormulaCompiler, e.g. in FormulaCompiler::Factor(), in ScInterpreter::Interpret(), in the 
functions themselves (ScInterpreter::ScIf() and ScInterpreter::ScIfError() or somewhere else?
And as these functions are jump functions, how best to access/modify the stack or FormulaToken?

Winfried