Sliding pieces moves are now illegal if after piece of same color

Sources/Engine/Square.swift

@@ -16,6 +16,25 @@ public struct Square: Equatable {
 			file = f
 		}
 
+		public init?(with rep: String) throws {
+			guard rep.count < 3 else {
+				throw Board.MoveFailure.squareANIsTooLong(an: rep)
+			}
+			guard rep.count > 1 else {
+				throw Board.MoveFailure.squareANIsTooShort(an: rep)
+			}
+			guard let f1 = rep.first, let r1 = rep.last, r1.isWholeNumber else {
+				return nil
+			}
+			guard
+				let srcFile = File.init(rawValue: String(f1))?.value
+			else {
+				return nil
+			}
+			rank = Int8(String(r1))!
+			file = srcFile
+		}
+
 		public enum File: String, CustomStringConvertible {
 			case a, b, c, d, e, f, g, h
 
@@ -70,6 +89,22 @@ public struct Square: Equatable {
 		public static func += (lhs: inout Position, rhs: (Int8, Int8)) {
 			lhs = lhs + rhs
 		}
+
+		public static func - (lhs: Position, rhs: Position) -> Position {
+			.init(rank: lhs.rank - rhs.rank, file: lhs.file - rhs.file)
+		}
+
+		public static func - (lhs: Position, rhs: (Int8, Int8)) -> Position {
+			.init(rank: lhs.rank - rhs.0, file: lhs.file - rhs.1)
+		}
+
+		public static func -= (lhs: inout Position, rhs: Position) {
+			lhs = lhs - rhs
+		}
+
+		public static func -= (lhs: inout Position, rhs: (Int8, Int8)) {
+			lhs = lhs - rhs
+		}
 	}
 
 	public let position: Position