A subscript triplet consists of three parts:  the first two parts
  designate a range of subscript values and the third part designates
  the increment (stride) between each value.  It takes the following
  form:

    [subscript-1] : [subscript-2] [:stride]

    subscript-1   Is a scalar numeric expression representing
                  the first value in the subscript sequence.
                  If omitted, the declared lower bound of the
                  dimension is used.

    subscript-2   Is a scalar numeric expression representing
                  the last value in the subscript sequence.
                  If omitted, the declared upper bound of the
                  dimension is used.

                  When indicating sections of an assumed-size
                  array, this subscript must be specified.

    stride        Is a scalar numeric expression representing
                  the increment between successive subscripts
                  in the sequence.  It must have a nonzero value.
                  If it is omitted, it is assumed to be 1.

  The stride has the following effects:

   o  If the stride is positive, the subscript range starts with the
      first subscript and is incremented by the value of the stride,
      until the largest value less than or equal to the second
      subscript is attained.

      For example, if an array has been declared as B(6,3,2), the
      array section specified as B(2:4,1:2,2) is a rank-two array
      with shape (3,2) and size 6.  It consists of the following six
      elements:

          B(2,1,2)   B(2,2,2)
          B(3,1,2)   B(3,2,2)
          B(4,1,2)   B(4,2,2)

      If the first subscript is greater than the second subscript,
      the range is empty.

   o  If the stride is negative, the subscript range starts with the
      value of the first subscript and is decremented by the absolute
      value of the stride, until the smallest value greater than or
      equal to the second subscript is attained.

      For example, if an array has been declared as A(15), the array
      section specified as A(10:3:-2) is a rank-one array with shape
      (4) and size 4.  It consists of the following four elements:

          A(10)
          A(8)
          A(6)
          A(4)

      If the second subscript is greater than the first subscript,
      the range is empty.

  If a range specified by the stride is empty, the array section has
  a size of zero.

  A subscript in a subscript triplet need not be within the declared
  bounds for that dimension if all values used to select the array
  elements are within the declared bounds.  For example, if an array
  has been declared as A(15), the array section specified as
  A(4:16:10) is valid.  The section is a rank-one array with shape
  (2) and size 2.  It consists of elements A(4) and A(14).

  If the subscript triplet does not specify bounds or stride, but
  only a colon (:), the entire declared range for the dimension is
  used.