I am surprised that this has happened to your son.
Those who have done it have been very unthinking and unbecoming.
They seem like delinquent elements rather normal members of civic society and I join you in denouncing such acts.
However I don't think this represents mainstream Singapore and would be very disappointed if your view is that it does.
I apologise on behalf of my countrymen if this has indeed happened to your son and hope that all those who have anything to do with these kinds of acts in Singapore will grow up and see the stupidity of it all.
It would be good if you can tell them when you see this happening as this will serve to tell the effect of their behaviour on others. Sincerely hope that this is an isolated incident and does not recur.
