The ultimate fate of that string is a mystery. I went through every turd for a fortnight---once she'd got over her reluctance to going back to using the litter tray, which meant at first my politely leaving the flat to save her blushes, can you believe it, is it the feline psychologist I am now?---and found no fibres other than cat hair. Eventually, and when she had mournfully given up any hope of seeing the garden again, the vet declared that she had probably got past the danger of strangling herself with her own intestine, or whatever, and she's been let back outside to shit it where she will.
One joyful result of all this spiritual oppression we've put her through is that she swallowed her worming pill today with almost no fuss at all.