's parameters to move the viewport.
These parameters are independent of the actual display location.

<P>
Viewport class assumes the origin of the viewport at the lower-left corner of
the rectangular area and y-axis extends to the upper direction.
Unfortunately, in many window systems and display devices, the origin is taken
at the upper-left corner and y-axis extends to the lower direction.
To work around this problem, a negative value should be given to the
<EM>:height</EM> parameter.

<P>

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