vavailabilityc = seq(/BEGIN:VAVAILABILITY(\r|\n|\r\n)/i.r, availabilityprops, availablec.star, /END:VAVAILABILITY(\r|\n|\r\n)/i.r) do |(_, e, a, _)|
          parse_err("Missing DTSTAMP property") unless e.has_key?(:DTSTAMP)
          parse_err("Missing UID property") unless e.has_key?(:UID)
          parse_err("Coocurring DTEND && DURATION properties") if e.has_key?(:DTEND) && e.has_key?(:DURATION)
          parse_err("Missing DTSTART property with DURATION property") if e.has_key?(:DURATION) && !e.has_key?(:DTSTART)