#
`-=`

(subtract assign) operator

The right-associative, binary infix `-=`

operator decrements it's left-hand operand
by the amount given by its right-hand operand.

## Usage

```
variable Integer num = 4;
num -= 1; // decrement num by 1
num -= num; // decrement num by 3
```

## Description

### Definition

The `-=`

operator is defined as follows:

```
lhs = lhs.minus(rhs)
```

except that `lhs`

is evaluated only once.

See the language specification for more details.

### Polymorphism

The `-=`

operator is polymorphic.
The definition of the `-=`

operator depends
on the `Invertible`

interface.

### Type

The result type of the `-=`

operator is the same as the type of its right hand operand.

## See also

- arithmetic operators in the language specification
- operator precedence in the language specification
- Operator polymorphism and Numeric operator semantics in the Tour of Ceylon