RETAIN_FS                    Field Separator

    TRUE/FALSE. default = FALSE.  If FALSE then OFS is used between  fields
    to reconstruct  $0 whenever  a field  value is  altered.   If TRUE  the
    original field separator characters  are retained in reconstructing  $0
    whenever a field value is altered.

    $0 == "This   string    contains     three       fields"

    $1 = "five";
    ==> $0 == "This string contains five fields"

    RETAIN_FS = TRUE;
    $1 = "five";
    ==> $0 == "This   string    contains     five       fields"