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Phil Ethridge2,736 Points
There's an pretty good example over here using math vs. code. If I remember from the course, this term was used to describe a String as a compound expression, so that might look something like this:
let fullName: String = firstName + " " + middleName + " " + lastName
Also a very basic example, but what makes it compound is that you're adding multiple types together. A regular expression, by contrast, would simply be:
let fullName: String = "John Smith"
Hope that helps!