A vector subscript is a rank-one array of integer values (within
  the declared bounds for the dimension).  It is used to select a
  sequence of elements from a parent array.  The sequence does not
  have to be in order, and it can contain duplicate values.

  For example, A is a rank-two array of shape (4,6).  B and C are
  rank-one arrays of shape (2) and (3), respectively, with the
  following values:

    B = (/1,4/)
    C = (/2,1,1/)         ! Will result in a many-one
                          !   array section

  Array section A(3,B) consists of elements A(3,1) and A(3,4).  Array
  section A(C,1) consists of elements A(2,1), A(1,1), and A(1,1).
  Array section A(B,C) consists of the following elements:

    A(1,2)   A(1,1)   A(1,1)
    A(4,2)   A(4,1)   A(4,1)

  An array section with a vector subscript that has two or more
  elements with the same value is called a many-one array section.  A
  many-one section must not appear on the left of the equals sign in
  an assignment statement, or as an input item in a READ statement.

  The following assignments to C also show examples of vector
  subscripts:

    INTEGER A(2), B(2), C(2)
    ...
    B    = (/1,2/)
    C(B) = A(B)
    C    = A((/1,2/))

  An array section with a vector subscript must not be any of the
  following:

   o  An internal file

   o  An actual argument associated with a dummy array that is
      defined or redefined (if the INTENT attribute is specified, it
      must be INTENT(IN))

   o  The target in a pointer assignment statement

  If the sequence specified by the vector subscript is empty, the
  array section has a size of zero.