Model definition:
-----------------

                                                                                                                 |~
class Party(metaclass=PoolMeta):                                                                                       |~
    __name__ = 'party.party'                                                                                           |~

    debtor = fields.Boolean("Debtor")                                                                                  |~
    debtor_icon = fields.Function(fields.Char("Debtor Icon"),                                                          |~
        'get_debtor_icon')

                                                                                                                       |~
    def get_debtor_icon(self, name):                                                                                   |~
        if self.debtor:                                                                                                |~
            return 'tryton-money-bag'                                                                                  |~
        return None

Tree view definition:
--------------------

<?xml version="1.0"?>
<!-- The COPYRIGHT file at the top level of this repository contains the full
    copyright notices and license terms. -->
<data>
    <xpath expr="/tree/field[@name='code']" position="before">
        <field name="debtor" icon="debtor_icon"/>
    </xpath>
</data>

Form view definition:
--------------------

<?xml version="1.0"?>
<!-- The COPYRIGHT file at the top level of this repository contains the full
    copyright notices and license terms. -->
<data>
    <xpath expr="/form/group[@id='checkboxes']" position="inside">
        <label name="debtor"/>
        <field name="debtor"/>
        <image name="tryton-money-bag" xexpand="0"/>
    </xpath>
</data>
~