Flags

• Bottom two bits hold type of SV
• Next two bits show which fields are valid

    $a = 3;		Type: Integer	Flags: SVf_IOK
     ... if $a eq "3";	Type: Int & Str	Flags: SVf_IOK, SVf_POK
    $a++;			Type: Int & Str	Flags: SVf_IOK
  

• SvFLAGS(sv) gets you the flags
  • SvFLAGS(a) &= ~SVf_POK
• Macros in sv.h for testing and setting
  • SvPOK_off(a)