human
Solve the following python programming problem given the following code:
def add_checkbox(self, layout, field, config_key, config_desc, config_val) : 
    cb = QtGui.QCheckBox (config_desc) 
    self.set_checkbox_value (cb, config_val) 
    row = layout.rowCount () 
    layout.addWidget (cb, row, 1) 
    def call_field_changed(val) : 
        self.field_changed (config_key, field, val, config_val) 
cb.toggled.connect (<MASK>) 
    return cb 
    
    Given the code above, what is a proper replacement for <MASK>?
assistant
#Here is a proper replacement for <MASK>.
def call_field_changed(