Differentials

Just had an injection, in the abdomen, which cost the French health service nearly 400€ a shot. The charming nurse who deftly and painlessly did the business charged me a mere 4€ and apologised profusely about the price she was obliged to make me pay for her services. She was the same one who took out the staples last winter. She recognised me straight away: 'You're the one who turned up with the money the next day when I forgot to charge you'. Must be a rare thing, round these parts, honesty...