one-rat rule.

The roof of the Biryani Emperor was held up by rafters of wood, and a rodent had materialized on one of them.
‘Boy!’ Masterji shouted.  ‘Look at that thing up there on the wood.’
The ‘boy’—the middle-aged waiter—looked up.  Undeterred by all the attention, the sly rat kept moving along the rafter, like a leopard on a branch.  The ‘boy’ yawned.
Masterji pushed his biryani, not even half eaten, in the direction of the boy.
‘I have a rule.  I can’t eat this.’
It was true: he had a ‘one-rat rule’—never revisit a place where a single rat has been observed.
‘You and your rule.’  Mr. Pinto helped himself to some of his friend’s biryani.
‘I don’t like competing for my food with animals.  Look at him up there: like a Caesar.’
‘A man has to bend his rules a little to enjoy life in Mumbai,’ Mr. Pinto said, chewing.  ‘Just a little.  Now and then.’

– Aravind Adiga, Last Man in Tower