ARGOMENTO: Cancellazione di un record di un campo o2m

from osv import osv
from osv import fields
import pdb

class hr_employee_skill(osv.osv):
    _name = "hr.employee.skill"
    _description = "Skill Risorsa"
    
    def _get_skill_complete(self, cr, uid, ids, field_name, arg, context=None):
        res = {}
        for record in self.browse(cr, uid, ids):
            skill = record.skill.complete_name
            level = record.level.x_name
            complete_skill = (skill + " " + level)
            res[record.id] = complete_skill
        return res
    
    def unlink(self, cr, uid, ids, context=None):
        pdb.set_trace()
        state = self.read(cr, uid, ids, ['is_primary'], context=context)
        for t in state:
            if t['is_primary']:
                raise osv.except_osv(_('Azione non consentita!'), _('Non puoi eliminare lo skill primario della risorsa.'))
            else:
                super(hr_employee_skill, self).unlink(cr, uid, ids, context)
        return True
    
    _columns = {
        'level': fields.many2one('x_senioritylevel', 'Seniority', select=True, required=True),
        'skill': fields.many2one('hr.employee.category', 'Skill', select=True, required=True),
        'name': fields.function(_get_skill_complete, method=True, type="char", string="Skill Primario", store=True),
        #'employee_ids': fields.many2many('hr.employee', 'employee_skills_rel', 'skill_id', 'emp_id', 'Risorse', store=True),
        'employee_id': fields.many2one('hr.employee', 'Risorsa', store=True, required=True, select=True),
        'is_primary': fields.boolean('Skill primario')
    }
    
    _order = "skill"

hr_employee_skill()

def unlink(self, cr, uid, ids, context=None):
    print 'BLABLA1'
    res = super(hr_employee_skill, self).unlink(cr, uid, ids, context)
    print 'BLABLA2'
    return res

def unlink(self, cr, uid, ids, context=None):
    lines = self.browse(cr, uid, ids, context=context)
    for line in lines:
        if line.is_primary:
            res = True    
            raise osv.except_osv(_('Azione non consentita!'), _('Non puoi eliminare lo skill primario della risorsa.'))
        else:
            res = super(hr_employee_skill, self).unlink(cr, uid, ids, context)
    return res

File "/opt/openerp/openerp-6.1-1/openerp/addons/hr_employee_skill/hr_employee_skill.py", line 55, in unlink
    raise osv.except_osv(_('Azione non consentita!'), _('Non puoi eliminare lo skill primario della risorsa.'))
NameError: global name '_' is not defined