Using the where clause when designing protocol-oriented APIs in Swift can let your implementations (or others' if it's open source) have a lot more freedom, especially when it comes to collections.

public protocol PathFinderMap {
    associatedtype Node
    // Using the 'where' clause for associated types, we can
    // ensure that a type meets certain requirements (in this
    // case that it's a sequence with Node elements).
    associatedtype NodeSequence: Sequence where NodeSequence.Element == Node

    // Instead of using a concrete type (like [Node]) here, we
    // give implementors of this protocol more freedom while
    // still meeting our requirements. For example, one
    // implementation might use Set<Node>.
    func neighbors(of node: Node) -> NodeSequence
}