is it possible to write a converter which only convert 0 to nil

[oyster]

The C code 
    
    
    int setCallback(Fl_Widget* wgt, Fl_Callback *cb, void *pdata=NULL, int 
num=1);
    
    
    Run

can be translated into 
    
    
    proc setCallback*(wgt: ptr Fl_Widget; cb: ptr Fl_Callback;
                                 pdata: pointer = nil; num: cint = 1): cint
    
    
    Run

Instead of 
    
    
    setCallback(wgt,  cb, nil, 2)
    
    
    Run

I want to call the function as 
    
    
    setCallback(wgt,  cb, 0, 2)
    
    
    Run

I have tried use 
    
    
    converter toNil(x: int=0): type(nil) = nil
    
    
    Run

but 
    
    
    Error: invalid type: 'nil' in this context: 'proc (x: int): nil' for proc
    
    
    Run

so, is it possible to write a convertor which actually convert number zero only 
to nil? thanks