function check_form(){
var errs=0;	
	id='verzend_form_knop';
	if(document.forms['grazielle_form'].elements['naam'].value == ''){
		errs += 1;		
	}	
	if(document.forms['grazielle_form'].elements['adres'].value == ''){
		errs += 1;		
	}
	if(document.forms['grazielle_form'].elements['postcode'].value == ''){
		errs += 1;		
	}	
	if(document.forms['grazielle_form'].elements['plaats'].value == ''){
		errs += 1;		
	}		
	if(document.forms['grazielle_form'].elements['telefoon'].value == ''){
		errs += 1;		
	}	
	if(document.forms['grazielle_form'].elements['email'].value == ''){
		errs += 1;		
	}		
	if(errs>0){
		var elem = document.getElementById(id);
		elem.disabled=true ;
	}
	else{
		var elem = document.getElementById(id);
		elem.disabled=false ;		
	}
}