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                                               161
      SECTION 3.8                                                  Common Data Structures



      numerical fields default to zero values. A plus sign (+) as the value of the O field
      signifies that local time is later than UT, a minus sign (−) signifies that local time
      is earlier than UT, and the letter Z signifies that local time is equal to UT. If no UT
      information is specified, the relationship of the specified time to UT is considered
      to be unknown. Regardless of whether the time zone is known, the rest of the date
      should be specified in local time.

      For example, December 23, 1998, at 7:52 PM, U.S. Pacific Standard Time, is
      represented by the string

         D : 199812231952 − 08 ' 00 '


3.8.4 Rectangles

      Rectangles are used to describe locations on a page and bounding boxes for a
      variety of objects, such as fonts. A rectangle is written as an array of four numbers
      giving the coordinates of a pair of diagonally opposite corners. Typically, the
      array takes the form

         [ llx lly urx ury ]

      specifying the lower-left x, lower-left y, upper-right x, and upper-right y
      coordinates of the rectangle, in that order. The other two corners of the rectangle
      are then assumed to have coordinates (llx, ury) and (urx, lly).

      Note: Although rectangles are conventionally specified by their lower-left and upper-
      right corners, it is acceptable to specify any two diagonally opposite corners. Appli-
      cations that process PDF should be prepared to normalize such rectangles in situa-
      tions where specific corners are required.


3.8.5 Name Trees

      A name tree serves a similar purpose to a dictionary—associating keys and
      values—but by different means. A name tree differs from a dictionary in the
      following important ways:

      • Unlike the keys in a dictionary, which are name objects, those in a name tree
        are strings.
      • The keys are ordered.

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