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This is a GeoGebra mini-tip for the command IsDefined,
which is a Boolean or true/false command that tells us
whether an object exists (is defined) or not.
So what is the easiest way to see this?

Let us draw 2 line segments A B and at this moment we
will have them cross C D. So they have an intersection
point here. So let us get the intersection point E.
What happens if we make it so that the segments do not
intersect. That is, we will click and drag D off and we see
that it says that E is undefined.
Drag it back. E is defined. So how can we make a
Boolean for that so that we can use that fact of whether
E is defined or not.
Click down in the input bar and start typing “is” and the
first command that comes up is IsDefined. So click on it.
And now we just type in the letter “capital E” and hit
enter. And momentarily c=true because E is defined. But
if we click and drag D off, c is now false.
So this Boolean says “yes” E is now defined or true. Or
“no” E is not defined or false.
That is it!