fires before the value of a cell is changed
| row | number | the row id |
| column | number | the column id |
| newValue | string | the new value |
| oldValue | string | the old value |
| boolean | returning false will prevent changing the value of a cell |
$$("ssheet").attachEvent("onBeforeValueChange",function(row,column,newValue,oldValue){
// your code here
return true;
});