rubocop: set max. line length to 140
rubocop: fixed "Keep a blank line before and after private"
rubocop: fixed "Extra empty lines"
rubocop: fixed "Nil comparisonwq"
rubocop: fixed "Comparison of something with itself"
rubocop: fixed "Style/TrivialAccessors" - allow predicates
rubocop: set Style/StringLiteralsInInterpolation to double_quotes
rubocop: fixed "Rename is_registered? to registered?"
rubocop: fixed "(...) interpreted as grouped expression"