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region<-area_x
region <-area_width: area -> int
region <-area_x: area -> int
region <-area_y: area -> int
region <-area_height: area -> int
Dimension of the described region for the specified area. Example: 
?- get(region(x+w/2, y+h/2, w/2, h/2), area_height,
           area(100, 100, 200, 200), H),

H = 100
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