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 ) ]