ANSWER OF LEVEL 21

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A physicist, a biologist, a mathematician and a programmer are sitting at a café across from an empty building. They observe two people enter and then, later, three leave.

The physicist says, "Apparently there was some error with our measurements."
The biologist says, "Obviously, they reproduced while in the building."
The mathematician opines, "If now one more were to enter the building, it would again be empty."

What did the programmer say?