def subset?(string)
      (Set.new(string.chars) - Set.new(charset)).empty?
    end
def subset?(string)
      (Set.new(string.chars) - Set.new(charset)).empty?
    end
Check if the provided string can be represented by the charset.

 Example

   charset = SecretSharing::Charset.by_charset_string "abc"
   charset.subset? "d"
   # => false
   charset.subset? "a"
   # => true

 @param string [String] Character to retrieve the for codepoint
 @return [TrueClass|FalseClass]