How (in ext js 2) to stub the method in the sources and rewrite it to your own?

Question

ExtJS/Sencha

How (in ext js 2) to stub the method in the sources and rewrite it to your own? For example in ext-all-debug.js I need to rewrite the method:
// private
doMultiSelection : function(item, index, e){
if(e.shiftKey && this.last !== false){
var last = this.last;
this.selectRange(last, index, e.shiftKey);
this.last = last; // reset the last
}else{
if((e.ctrlKey||this.simpleSelect) && this.isSelected(index)){
this.deselect(index);
}else{
this.select(index, e.ctrlKey || e.shiftKey || this.simpleSelect);
}
}
},