add_insn_after() abort question

[Toshiyasu Morita]

I have some code which causes gcc to abort in the following code:

/* Add INSN into the doubly-linked list after insn AFTER.  This and
   the next should be the only functions called to insert an insn once
   delay slots have been filled since only they know how to update a
   SEQUENCE.  */

void
add_insn_after (insn, after)
     rtx insn, after;
{
  rtx next = NEXT_INSN (after);

  if (optimize && INSN_DELETED_P (after))
    abort (); <- here

I'm trying to understand why it's not legal to add an insn after a 
deleted insns when optimizing?

Toshi