I don't know exactly what you're asking for, but I'll try to help you. :)

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Thank you, Pauan.

Ok. I made a mistake not to call apply.

Now it is clear. I learned from this mistake.

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The phrase "apply it" is confusing because we sometimes say "apply a function (to some arguments)" even when we're not calling apply.

As a verb, "to mistake something (for something else)" means "to misunderstand something (thinking it's something else instead)." As a noun, "a mistake" is an accidental occurrence. The idiom "to make a mistake" means to cause an occurrence by accident.