How to do math operations with model fields or expressions in Agile Toolkit
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atk4.2.1

I have this model:

    class Model_Cargo extends Model_Table {
    public $table='cargo';
    function init(){
        parent::init();
        
        $this->hasOne('Alumno');
        $this->hasOne('Plan');
        
        $this->addField('fecha')->type('date');
        $this->addField('fechaCreacion')->type('date');
        $this->addField('fechaVencimiento')->type('date');
        $this->addField('name');
        $this->addField('monto')->type('money');
        $this->addField('cancelado')->type('boolean')->defaultValue(false);
               
        $this->hasMany('Abono');
        $this->addExpression('abonos')->set($this->refSQL('Abono')->sum('monto'));
       }
    }
I wanto to make a math operation + or - with two fields:
I actually want to substracr field 'monto' with expression 'abonos' how do I do it?

lets say something like this:

    $this->addExpression('balance')->set('monto'-'abonos');
    //this does not work


I also wold like to addCondition where those fields are equal... can I do that?

something lke:

    $this->addCondition('monto','abonos');
    //this does not work