Editing Area
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   This is were it all happens. This is were the entered expression is
displayed, as you enter it. Expressions is entered using the  Keys and
the previously described gadgets. It is also possible to use the mouse
for placing the cursor or marking blocks. Clicking somewhere moves the
cursor to symbol you clicked on, while pressing the left button
somewhere and then drag the mouse across some of the expression will
mark these, just as if you used the  Mark menu.