Replace Operand with Operator

Sources/PMCalc/Operator.swift

@@ -1,4 +1,4 @@
-enum Operand {
+enum Operator {
     case add
     case sub
     case mul

Sources/PMCalc/ViewModel.swift

@@ -5,7 +5,7 @@ struct ViewModel {
     private var numberBuffer: String = ""
     private var number1: Double? = nil
     private var number2: Double? = nil
-    private var operand: Operand? = nil
+    private var op: Operator? = nil
     @State
     private(set) var display: String = ""
 
@@ -13,7 +13,7 @@ struct ViewModel {
         numberBuffer = ""
         number1 = nil
         number2 = nil
-        operand = nil
+        op = nil
         display = ""
     }
 
@@ -31,18 +31,18 @@ struct ViewModel {
         display = numberBuffer
     }
 
-    mutating func operation(_ operand: Operand) {
+    mutating func operation(_ op: Operator) {
         guard !numberBuffer.isEmpty && number1 == nil else { return }
 
-        self.operand = operand
+        self.op = op
         if number1 == nil { number1 = Double(numberBuffer) }
         else { number2 = Double(numberBuffer) }
         numberBuffer = ""
 
-        if operand == .sqrt {
+        if op == .sqrt {
             display = String(sqrt(number1!))
             number1 = nil
-            self.operand = nil
+            self.op = nil
         }
     }
 
@@ -52,7 +52,7 @@ struct ViewModel {
 
         var result: Double = 0.0
 
-        switch operand {
+        switch op {
         case .add:
             result = add_nums(number1!, number2!)
         case .sub: