NULLIFY

 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.