class NameOfPerson::PersonName

def possessive(method = :full)

Returns full name with with trailing 's or ' if name ends in s. 
def possessive(method = :full)
  whitelist = %i[full first last abbreviated sorted initials]
  unless whitelist.include?(method.to_sym)
    raise ArgumentError, 'Please provide a valid method'
  end
  name = public_send(method)
  @possessive ||= "#{name}'#{'s' unless name.downcase.end_with?('s')}"
end