@RoelDriessen, here UKHeliBob and me have different opinions.
With the Arduino, false = 0, LOW = 0, INPUT = 0 and true = 1, HIGH = 1 and OUTPUT = 1.
I like to keep them seperated.
The boolean "not" operator is the "!", but I use that for 'bool' variables only.
A "not" with a "HIGH" or "LOW" has no meaning for me.
You can decide for yourself what suits you best.