Make public _domain_column, _domain_value and _domain_add_null from Field
Those methods are already used in modules:
$ ack _domain_value modules/
modules/bank/bank.py
108: expression = Operator(column, field._domain_value(operator, value))
modules/product_cost_fifo/move.py
54: expression = Operator(column, field._domain_value(operator, value))
modules/sale_subscription/subscription.py
729: expression = Operator(column, field._domain_value(operator, value))
And they are useful to implement a domain_<field>
method.