    # File lib/prawn/document/page_geometry.rb, line 139
          def page_dimensions
            coords = SIZES[page_size] || page_size
            [0,0] + case(page_layout)
            when :portrait
              coords
            when :landscape
              coords.reverse
            else
              raise Prawn::Errors::InvalidPageLayout,
                "Layout must be either :portrait or :landscape"
            end
          end