Validate field values on form

A nifty trick for comparing the "old" value against the "new" value using "obj.orig()" when validating a field on a form.
Example:
Change the behavior of the "Customer Group" on "CustTable" form.
Overwrite the method "validate" on the field "CustGroup" on the data source on "CustTable" public boolean validate()
{
boolean ret;

ret = super();

return ret;
}
Change method:

public boolean validate()
{
boolean ret;
;
if(custTable.orig().CustGroup == '30') // Non changeable group.
{
info('You cannot change this Customer group');
ret = false;
} else {
ret = super();
}
return ret;

}
Note: I've explained the general principle for validating a field against it's old value, however there are some issues with the data source not refreshing data set when changing the value back and forth.