"If the distinction is worth making, McG, it has to work all the time. In speech as well as in writing. " Why? Speaking and writing are different forms of communication, with all kinds of different conventions. For example you can't ask a written sentence to clarify an ambiguity.
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Never get tempted to go into the shop and point out the solecism to the shopkeeper, Schantieman. Maybe in the course of making a purchase - that's how it works.