DEBUG Variable?

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Picklesworth

I seem to have a variable/operator/something in blitz called DEBUG...
According to Blitz, it is not a function, but it gets highlighed. When accessed as a variable, it seems to always equal 0.

Open in online IDE

Graphics 640,480,0,2
While Not KeyDown(1)
	i = DEBUG
	Print "??? "+i
Wend
End

What the heck is this?

Edit:
It's not a function for sure - I can change its value with DEBUG=n
Further testing tells me that it is an integer, because it will automatically convert from a float to an integer, and when I try to give it a string value it will just equal 0.

Edit again:
My bad. Still weird, but I thought it was only an integer because I suddenly forgot at the time to define the variable...
Still weird though; why would it be highlighted?