SwiftExample.swift

  private mutating func performPendingBinaryOperation() {
      if pbo != nil && accumulator != nil {
          accumulator = pbo!.perform(with: accumulator!)
          pbo = nil
      }
  }

  private var pbo: PendingBinaryOperation?

  private struct PendingBinaryOperation {
      let function: (Double, Double) -> Double
      let firstOperand: Double

      func perform(with secondOperand: Double) -> Double {
          return function(firstOperand, secondOperand)
      }
  }

  mutating func setOperand(_ operand: Double) {
      accumulator = operand
  }

  var result: Double? {
      get {
          return accumulator
      }
  }