%=
From Microduino Wiki
%=(Compound modulo)
*Description
The arithmetic operator used among variables and constants. "%=" is a way of abbreviation.
*Syntax
x %= y; // It is equal to x = x % y;
*Parameters
x: Variable in non floating-point.
y: Variable or constant in non floating-point.
*For example
x = 9; x %= 4; // x = 1
*Note
The modulus operator cannot be used for floating-point data.