There is a standard method

 def read_group(self, cr, uid, domain, fields, groupby, offset=0, limit=None, context=None, orderby=False)

I'm calling a super on it and passing a domain like this

[('end_date', '>=', '2019-05-01'), ('end_date', '<=', '2019-05-30'), ('employee', 'in', [49]), ('state', 'in', ['done'])]

and it works.

but when I try to append

domain.append(('&'))
domain.append(('state', 'in', ['progress']))

I get

File "/home/www/workspace/openerp-7.0-20140804-231303/openerp/osv/expression.py", line 201, in normalize_domain
    assert expected == 0, 'This domain is syntactically not correct: %s' % (domain)
AssertionError: This domain is syntactically not correct: [('end_date', '>=', '2019-05-01'), ('end_date', '<=', '2019-05-30'), ('employee', 'in', [49]), ('state', 'in', ['done']), '&', ('state', 'in', ['progress'])]

basically, need to filter all records by dates and in state 'done' and on top of that all records in state 'progress'.