def build(self, builder):
        """Build XML by appending to builder"""

        params = dict(
            OID=self.oid,
            Name=self.name,
            Repeating=bool_to_yes_no(self.repeating),
            Type=self.event_type,
        )

        if self.category is not None:
            params["Category"] = self.category

        if self.access_days is not None:
            params["mdsol:AccessDays"] = str(self.access_days)

        if self.start_win_days is not None:
            params["mdsol:StartWinDays"] = str(self.start_win_days)

        if self.target_days is not None:
            params["mdsol:TargetDays"] = str(self.target_days)

        if self.end_win_days is not None:
            params["mdsol:EndWinDays"] = str(self.end_win_days)

        if self.overdue_days is not None:
            params["mdsol:OverDueDays"] = str(self.overdue_days)

        if self.close_days is not None:
            params["mdsol:CloseDays"] = str(self.close_days)

        builder.start("StudyEventDef", params)
        for formref in self.formrefs:
            formref.build(builder)
        for alias in self.aliases:
            alias.build(builder)
        builder.end("StudyEventDef")