LET /WHERE

        [SIC\]LET Variable = Expression /WHERE Condition_mask

    The LET command allows setting variables only where  a  given  condition
    mask  is  .TRUE.. The condition mask can be a logical array or a logical
    expression (of same dimension as the result  variable).  Implicit  loops
    can be used in conjunction to the /WHERE option.

    For example
       DEFINE REAL A[4,5] B[4,5]
       LET B[I,J] = I+J
       LET A[I,J] = (I-J)**2 /WHERE COS(I).GT.SIN(J)  ! 1
       LET A[I,J] = SIN(I+J) /WHERE B.GT.5            ! 2
    is  equivalent  (in terms of results, but about 500 times faster) to the
    loops
       DEFINE REAL A[4,5] B[4,5]
       LET B[I,J] = I+J
    ! 1
       FOR J 1 to 5
         FOR I 1 to 4
           IF (COS(I).GT.SIN(J)) THEN
             LET A[I,J] = (I-J)**2
           ENDIF
         NEXT
       NEXT
    ! 2
       FOR J 1 to 5
         FOR I 1 to 4
           IF (B[I,J].GT.5) THEN
             LET A[I,J] = SIN(I+J)
           ENDIF
         NEXT
       NEXT

    Note in the example above that implicit variables can be used.  However,
    the following syntax is non valid,
       LET A[I,J] = SIN(I+J) /WHERE B[I,J].GT.5
    because  implicit variables cannot appear as indexes to an operand array
    (B), but only as indexes to the result array (A) or as variables  as  in
    SIN(I+J). The correct syntax would be
       LET A[I,J] = SIN(I+J) /WHERE B.GT.5
    The following syntax is also non valid
       LET A = B /WHERE B[I,J].GT.LOG(I+J)
    because  implicit  variables  cannot be defined by an operand array (B),
    but only by the result array. The correct syntax would be
       LET A[I,J] = B /WHERE B.GT.LOG(I+J)