operator: list-to-words

    description:
        list-to-words sp
        convert the given superposition into a comma-separated word list
        it is the inverse of words-to-list

    examples:
        list-to-words |x>
            |x>
      
        list-to-words (|x> + |y>)
            |x and y>

        list-to-words (|x> + |y> + |z>)
            |x, y and z>
        
        list-to-words (|a> + |b> + |c> + |d> + |e>)
            |a, b, c, d and e>

        friends |Eric> => |Sam> + |Harry> + |Mary> + |Liz>
        list-to-words friends |Eric>
            |Sam, Harry, Mary and Liz>
    
        -- demonstration of inverse property:
        words-to-list list-to-words (|a> + |b> + |c> + |d> + |e>)
            |a> + |b> + |c> + |d> + |e>
      
    future:
        maybe also have an 'or' version too: 'x, y or z'
        handle sequences better

    see also:
      words-to-list