Writing textbooks has got to be pretty tedious work. So you can hardly blame the writers when they slip in something that seems a little bit… off. My theory is that one of three things happens:
#1. The writer slips something in to see if anybody notices.
#2. The writer is so focused on the topic at hand that he/she forgets common sense.
#3. The writer blacks out and temporarily loses his/her mind.
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