anchor.diff:  ;;;
   ;;;      Internal class. Its purpose is to hold cached precomputed RGBA array
   ;;;      for other patterns (so we collapse designs used as inks and opacities
  -;;;      into their final values). Computing such array is not necessarily
  +;;;      into their final values). Computing such an array is not necessarily
   ;;;      trivial, for instance an INDEXED-PATTERN may have a RECTANGULAR-TILE as
  -;;;      one of its designs, in which case we "blit" retangular tile instead of
  +;;;      one of its designs, in which case we "blit" rectangular tile instead of
   ;;;      simple color to all original array elements pointing at the tile. In
   ;;;      case of transformations this pattern should contain final (possibly
   ;;;      interpolated) values. This instance may be computed lazily and cached.
  
positive.code: ;;;    implement bounding-rectangle protocol. In case of rectangular-tile it
  ;;;    should work on its base design size (and a transformation).
  ;;;
  ;;; A: This is implied by the fact that it is adviced to use transform-region on
  ;;;    a pattern in order to transform it. See 14.5.
  ;;;
  ;;; Q: Should a pattern which is not transformed have a starting position?
  
negative.code: ;;;;
  ;;;;    License:  LGPL-2.1-or-later
  
  ;;; Patterns are a bounded rectangular arrangements of designs, like a
  ;;; checkboard. Pattern may be transformed and composed with other designs.
  ;;;
  ;;; Extensions:
  