Disassociates a pointer from its target.  It takes the following
  form:

     NULLIFY (ptr-obj [,ptr-obj]...)

     ptr-obj  Is a structure component or the name of a
              variable; it must be a pointer (have the
              POINTER attribute).

  The initial association status of a pointer is undefined.  You can
  use NULLIFY to initialize an undefined pointer, giving it
  disassociated status.  Then the pointer can be tested using the
  intrinsic function ASSOCIATED.

  EXAMPLES:

     REAL, TARGET  :: TAR(0:50)
     REAL, POINTER :: PTR_A(:), PTR_B(:)
     PTR_A => TAR
     PTR_B => TAR
     ...
     NULLIFY(PTR_A)

  After these statements are executed, PTR_A will have disassociated
  status, while PTR_B will continue to be associated with variable
  TAR.