sub isdual {
    my( $v ) = @_;
    no warnings;
    return 0
        if  int $v ne $v^'0'    # No IV nor NV in value
        ||  $v eq 0+$v          # Cached string just same as IV/NV
        ||  $v == "$v"          # Cached number just same as 0+PV
    ;
    return 1;
}