‐view update
 ‐each table/view satisfies a predicate
 ‐the definition is actually a set function defining the view
 ‐set functions do not need to be injective
 ‐when they’re not, they cannot be inverted even from their image
 ‐view update is equivalent to inversion, so it cannot be done in general
 ‐homomorphism theorems can yield consistent update rules for any finite,
  static set of views, but this won’t be a full inversion