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    <table name="foo">
        <field name="a" required="1"/>
        <field name="b" validate="\d+\.\d+\.\d+"/>
        <field name="c" required="if a"/>
    </table>

    package Model::Validate::Foo;

...
            die "c is required if a" if $value_of{a};
        }
    }

    <table name="foo">
        ...
        <constraint name="avg_a" value="average(a) > 100"/>
    </table>

    sub avg_a {
        # called on insert, update of table foo
        # check if average(a) > 100
    }