(model) changed signature of Entity#update to pass a list of expressions
It is much simpler this way. I use a list of expressions rather than a
list of arguments (as available from Instance) because it is easier to
manipulate. And if necessary:
  1) List<Argument> can be mapped to List<Expression> with
Lists.transform and a Function<Argument,Expression> (thanks Guava)
  2) an Argument that corresponds to an expression can be accessed by
(Argument) expression.eContainer()