define gravatar(email::string,-rating::string='',-default::string='',-size::integer=80) => {
//    Retrieves the Gravatar for the given email address.
 
	fail_if( 
		!valid_email(#email), 
		-1, 'Email address supplied is not valid.' 
	)
 
	fail_if( 
		#rating->size && array('G','PG','R','X') !>> #rating, 
		-1, 'Rating must be one of G, PG, R, or X.' 
	)
	
	fail_if( 
		#size < 1 || #size > 80, 
		-1, 'Size must be between 1 and 80.' 
	)
     
	fail_if( 
		#default->size && !valid_url(#default), 
		-1, 'Default must be a complete, valid URL.' 
	)
 
    local(hash = encrypt_md5(#email))
    local(url = 'http://www.gravatar.com/avatar.php?gravatar_id=' + #hash)
    #rating->size ? #url += '&rating=' + #rating
	#url += '&size=' + #size
	#default->size ? #url += '&default=' + encode_stricturl(#default)
    return(#url)     
}