def create_room_pairs(session_capacities):
    """
    Creates all size 2 combinations of rooms.
    Parameters: 
        session_capacities: a dictionary mapping each session name to a positive
                        integer for how many students can fit in that session
    Returns a set of room pairings (tuples).
    """    
    
    sessions = [s for s in session_capacities]
    pairs = {(sessions[i], j) for i in range(len(sessions)) for j in sessions[i:] if sessions[i] != j}
    return pairs