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A program starts at main, and main says what it may touch

This tour runs hd in your browser. Press Run, or Ctrl+Enter (Cmd+Enter on a Mac), to compile and run the code on the right. Edit it freely: your changes stay in this browser, and Reset brings back the original.

$ Console after the signature is a requirement. It is the whole list of outside things this program may use: it may print, and nothing else. No library it calls can quietly ask to read your files or open a socket unless main grants it, so a reviewer learns what the code asks for from one line. A row bounds what code asks for, not what a value you pass it already carries.

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