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        akeys           keys on array           ck_each         t%      A
        avalues         values on array         ck_each         t%      A
        each            each                    ck_each         %       H
        values          values                  ck_each         t%      H
        keys            keys                    ck_each         t%      H
        delete          delete                  ck_delete       %       S
        exists          exists                  ck_exists       is%     S
        pop             pop                     ck_shift        s%      A?
        shift           shift                   ck_shift        s%      A?
        caller          caller                  ck_fun          t%      S?
        reset           symbol reset            ck_fun          is%     S?
        exit            exit                    ck_exit         ds%     S?
        ...

=head2 FILESTATOP

A FILESTATOP may be a L</UNOP>, L</PADOP>, L</BASEOP> or L</SVOP>.

It has the class signifier B<->.

The file stat OPs are created via UNI(OP_foo) in toke.c but use the
C<OPf_REF> flag to distinguish between OP types instead of the usual
C<OPf_SPECIAL> flag. As usual, if C<OPf_KIDS> is set, then we return
C<OPc_UNOP> so that C<walkoptree> can find our children. If C<OPf_KIDS> is not
set then we check C<OPf_REF>. Without C<OPf_REF> set (no argument to the
operator) it's an OP; with C<OPf_REF> set it's an SVOP (and the field C<op_sv> is the
GV for the filehandle argument).

  case OA_FILESTATOP:
	return ((o->op_flags & OPf_KIDS) ? OPc_UNOP :
  #ifdef USE_ITHREADS
		(o->op_flags & OPf_REF) ? OPc_PADOP : OPc_BASEOP);
  #else
		(o->op_flags & OPf_REF) ? OPc_SVOP : OPc_BASEOP);
  #endif


        lstat           lstat                   ck_ftst         u-      F
        stat            stat                    ck_ftst         u-      F
        ftrread         -R                      ck_ftst         isu-    F-+
        ftrwrite        -W                      ck_ftst         isu-    F-+
        ftrexec         -X                      ck_ftst         isu-    F-+
        fteread         -r                      ck_ftst         isu-    F-+
        ftewrite        -w                      ck_ftst         isu-    F-+
        fteexec         -x                      ck_ftst         isu-    F-+
        ftis            -e                      ck_ftst         isu-    F-
        ftsize          -s                      ck_ftst         istu-   F-
        ftmtime         -M                      ck_ftst         stu-    F-
        ftatime         -A                      ck_ftst         stu-    F-
        ftctime         -C                      ck_ftst         stu-    F-
        ftrowned        -O                      ck_ftst         isu-    F-
        fteowned        -o                      ck_ftst         isu-    F-
        ftzero          -z                      ck_ftst         isu-    F-
        ftsock          -S                      ck_ftst         isu-    F-
        ftchr           -c                      ck_ftst         isu-    F-
        ftblk           -b                      ck_ftst         isu-    F-
        ftfile          -f                      ck_ftst         isu-    F-
        ftdir           -d                      ck_ftst         isu-    F-
        ftpipe          -p                      ck_ftst         isu-    F-
        ftsuid          -u                      ck_ftst         isu-    F-
        ftsgid          -g                      ck_ftst         isu-    F-
        ftsvtx          -k                      ck_ftst         isu-    F-
        ftlink          -l                      ck_ftst         isu-    F-
        fttty           -t                      ck_ftst         is-     F-
        fttext          -T                      ck_ftst         isu-    F-
        ftbinary        -B                      ck_ftst         isu-    F-

=head2 LOOPEXOP

A LOOPEXOP is almost a L<BASEOP_OR_UNOP>. It may be a L</UNOP> if stacked or 
L</BASEOP> if special or L</PVOP> else.

C<next>, C<last>, C<redo>, C<dump> and C<goto> use C<OPf_SPECIAL> to indicate that a
label was omitted (in which case it's a L</BASEOP>) or else a term was
seen. In this last case, all except goto are definitely L</PVOP> but
goto is either a PVOP (with an ordinary constant label), an L</UNOP>
with C<OPf_STACKED> (with a non-constant non-sub) or an L</UNOP> for
C<OP_REFGEN> (with C<goto &sub>) in which case C<OPf_STACKED> also seems to
get set.

...

=head2 OP Definition Example

Let's take a simple example for a opcode definition in F<opcode.pl>:

  left_shift	left bitshift (<<)	ck_bitop	fsT2	S S

The op C<left_shift> has a check function C<ck_bitop> (normally most ops 
have no check function, just C<ck_null>), and the options C<fsT2>.
The last two C<S S> describe the type of the two required operands: 
SV or scalar. This is similar to XS protoypes.
The last C<2> in the options C<fsT2> denotes the class BINOP, with 
two args on the stack.
Every binop takes two args and this produces one scalar, see the C<s> flag.
The other remaining flags are C<f> and C<T>.

C<f> tells the compiler in the first pass to call C<fold_constants()> 
on this op. See L</"Compile pass 1: check routines and constant folding">
If both args are constant, the result is constant also and the op will 
be nullified.

Now let's inspect the simple definition of this op in F<pp.c>.
C<pp_left_shift> is the C<op_ppaddr>, the function pointer, for every 
left_shift op.

  PP(pp_left_shift)
  {
    dVAR; dSP; dATARGET; tryAMAGICbin(lshift,opASSIGN);
    {
      const IV shift = POPi;
      if (PL_op->op_private & HINT_INTEGER) {
	const IV i = TOPi;
	SETi(i << shift);
      }
      else {
	const UV u = TOPu;
	SETu(u << shift);
      }
      RETURN;
    }