module Enumerable

def sole

{ a: 1, b: 2 }.sole # => Enumerable::SoleItemExpectedError: multiple items found
Set.new.sole # => Enumerable::SoleItemExpectedError: no item found
["x"].sole # => "x"

than one item, raises +Enumerable::SoleItemExpectedError+.
Returns the sole item in the enumerable. If there are no items, or more 
def sole
  case count
  when 1   then return first # rubocop:disable Style/RedundantReturn
  when 0   then raise ActiveSupport::EnumerableCoreExt::SoleItemExpectedError, "no item found"
  when 2.. then raise ActiveSupport::EnumerableCoreExt::SoleItemExpectedError, "multiple items found"
  end
end