similar to tolog, Prolog predicates

....
WHERE
   is-composed-by (opera: $opera, composer: puccini, ...)
      

this is actually only a shortcut too:

....
WHERE
   // is-composed-by [ . -> opera = $opera ] [ . -> composer = puccini ]

non-strict vs. strict predicates

=> allow additional roles to exist or not

   is-composed-by (opera: $opera, composer: puccini)
=  // is-composed-by [ . -> opera = $opera ]
                     [ . -> composer = puccini ]
                     [ not . roles :>: - ( opera ) - ( composer ) ]