(SET-VIEWPORT upper-left-point lower-right-point clip?)

Translates the coordinate system so that the coordinates of screen upper-left corner before a call are the same as the coordinates of upper-left-point after the call; moves the pen to this point; if clip? is #T or 1, clip all incoming function calls to the rectangle between the two points (i.e. no drawing will occur outside of this zone).

The redefinition of viewport is always relative to the whole screen, using the global (user- or not) coordinates system.