Merger doctrine
The rule that an expression loses copyright protection when the idea can only be expressed in one or a handful of ways.
If protecting the wording would in practice mean protecting the idea, the two have "merged" and neither gets a monopoly. The classic case is Morrissey v. Procter & Gamble from 1967, where the rules of a sweepstakes contest were judged too simple to be written in more than a few ways.
In software it is one of the most important filters. The Altai test strips out code dictated by efficiency, and in 2012 Judge William Alsup leaned on merger reasoning to clear Google's copying of Java's method declarations — there are only so many ways to write max(int a, int b). The Federal Circuit disagreed in 2014: Sun had had plenty of other names to choose from when the API was first written.