A question about weak contract

Hello!!

I'm reading a paper from a university and it says.

System boundary classes need weak contracts.
- The user might not be trustable.
* End-users do not accept that they have to test before a function call,
which calls for weak contract.

Now to my question why do we have to use weak contract here as was said.
Why is it not possible to use strong contract in that situation?
Can somebody explain that.

//Tony




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