core-data - Swift 3 Core Data - NSExpression 结合多种功能

Question

我需要这个表达式 --> sum:(abs:(amount) * -1)

我的代码是：

let absExpression = NSExpression(forFunction: "abs:", arguments: [NSExpression(forKeyPath: Table.COLUMN_AMOUNT)])
let minusAbsExpression = NSExpression(format: "%@ * -1", argumentArray: [absExpression])
let trueExpression = NSExpression(forFunction: "sum:", arguments: [minusAbsExpression])

但崩溃的原因是：

reason: 'Unsupported function expression sum:(abs:(amount) * -1)'