class RuboCop::Cop::Style::WhenThen

def on_when(node)
  return if node.multiline? || node.then? || !node.body
  message = format(MSG, expression: node.conditions.map(&:source).join(', '))
  add_offense(node.loc.begin, message: message) do |corrector|
    corrector.replace(node.loc.begin, ' then')
  end
end