                         multiplier: multiplier,
                         constant: constant
                     )
+                @unknown default:
+                    assertionFailure("Unknown relation '\(relation)', falling back to `.equal`.", file: file, line: line)
+                    return anchor.constraint(
+                        equalTo: space,
+                        multiplier: multiplier,
+                        constant: constant
+                    )
                 }
             })
     }

                case .greaterThanOrEqual:
                    return firstAnchor.constraint(
                        greaterThanOrEqualToSystemSpacingBelow: secondAnchor,
                        multiplier: multiplier
                    )
                }

                        greaterThanOrEqualToSystemSpacingAfter: secondAnchor,
                        multiplier: multiplier
                    )
                @unknown default:
                    assertionFailure("Unknown relation '\(relation)', falling back to `.equal`.", file: file, line: line)
                    return firstAnchor.constraint(
                        equalToSystemSpacingAfter: secondAnchor,
                        multiplier: multiplier
                    )
                }
            })
    }
