程式易讀－笛摩根定律

1) not (a or b or c) => (not a) and (not b) and (not c)
2) not (a and b and c) => (not a) or (not b) or (not c)

Example:
if ( !(file_exists && !is_protected)) { ... } => if ( !file_exists or is_protected) { ... }