-=
From Microduino Wiki
-=(Compound subtraction)
*Description
Arithmetic operator used between variables or constants. "-=" is an abbreviation.
*Syntax
x += y; // It is equal to x = x + y; x -= y; // It is equal to x = x - y; x *= y; //It is equal to x = x * y; x /= y; // It is equal to x = x / y;
*Parameters
x: variable in any data type
y: variable or constant in any data type
*Example
x = 2; x += 4; // x = 6 x -= 3; // x = 3 x *= 10; // x = 30 x /= 2; // x = 15