Synopsis:
   $common(<word list> / <word list>)
   $diff(<word list> / <word list>)

Technical:
   These functions are exact opposites.  The $common() function finds and
   returns all words common to both input lists.  The $diff() function
   funds are returns all words present in only one list.  The two input
   lists must be separated by a single forward-slash.  Whitespace is not
   significant.

Practical:
   These functions might be considered useful for statistical information.
   For instance, given 2 channels, they could be used to show who is and
   isn't on both of them.

Returns:
   common: words in both lists
   diff:   words not in both lists

Examples:
   $common(one two three / two three four)     returns "two three"
   $common(one two / three four)               returns ""  (empty string)
   $diff(one two three / two three four)       returns "one four"
   $diff(one two / one two)                    returns ""  (empty string)

See Also:
   common(6); remws(6); uniq(6)