feature: adds support for Quantity predecession

Author: bjacobgordon

NumberSystems/Sources/NumberSystems/Operable.swift

@@ -1,6 +1,9 @@
 public protocol Operable {
     /// Changes the cardinality of this value to the next one.
     mutating func succeed()
+    
+    /// Changes the cardinality of this value to the previous one.
+    mutating func precede()
 }
 
 extension Operable {
@@ -9,4 +12,10 @@ extension Operable {
         clone.succeed()
         return clone
     }
+    
+    public var predecessor: Self {
+        var clone = self
+        clone.precede()
+        return clone
+    }
 }

NumberSystems/Sources/NumberSystems/Quantity.swift

@@ -36,6 +36,12 @@ extension Quantity: Operable {
         let newElement = self.embodiment.first ?? Quantity.standardEmbodyingElement
         self.embodiment.append(newElement)
     }
+    
+    public mutating func precede() {
+        guard (self != Quantity.none) else { fatalError("There is no precedent for a lack of quantity") }
+        
+        self.embodiment.removeLast()
+    }
 }
 
 extension String {

NumberSystems/Tests/NumberSystemsTests/QuantityTests.swift

@@ -71,3 +71,16 @@ func successorOfQuantity(_ givenCount: Int) async throws {
     let succeededQuantity = operativeQuantity.successor
     #expect(succeededCount.represents(succeededQuantity))
 }
+
+@Test("Predecessor of quantities", arguments: [
+    1,
+    10,
+    100,
+    1000
+])
+func predecessorOfQuantity(_ givenCount: Int) async throws {
+    let operativeQuantity = givenCount.asQuantity
+    let  precededCount    = givenCount - 1
+    let  precededQuantity = operativeQuantity.predecessor
+    #expect(precededCount.represents(precededQuantity))
+}