How to create entity-relationship diagram for tables that can potentially be united by an "IS A" relationship?

Question

I have this schema:

create table hall(
  building-id int,
  hall-id int,
  capacity int,
  primary key(building-id, hall-id)
);

create table semester(
  sem-id char(5),
  start-date date,
  eend-date date,
  primary key(sem-id)
);

create table weekly-event(
  weid int,
  course-id numeric(7,0),
  sem-id char(5),
  weekday varchar(10),
  start-time time,
  duration interval,
  registered int,
  primary key(weid),
  foreign key(sem-id) references semester
);

create table one-time-event(
  oeid int,
  event-name varchar(50),
  end-date date,
  start-time time,
  duration interval,
  registered int,
  primary key(oeid)
);

create table timetable(
  start-date date,
  start-time time,
  hall-id int,
  building-id int,
  event-id int,
  duration interval,
  primary key(start-date, start-time, hall-id, building-id, event-id),
  foreign key(hall-id, building-id) references hall
);

where either events that occur on a weekly basis or one-time events are scheduled on some campus. I want to draw entity-relationship diagram (ERD) for this schema and first of all I don't have much experience in drawing ERDs.

Also, I'm wondering if I should use some kind of "IS A" relationship for the weekly and one-time events and unite some attribute under event table which would have children which would contain only the attributes specific to them.

This is the ERD I have so far:

As far as I see it, timetable is dependent on other tables that's why I chose rhombuses with double lines. I also think that the relationship between timetable and hall and weekly-events is one-to-many while the relationship between timetable and one-time-event is one-to-one.

Because weekly-event references semester it has total participation. Also logical timetable cannot exist without at least one hall, one-time-event or weekly-event that's why it also has total participation in those.