lower case = abstract
upper case = instantiatable
| = or
((xxx)) = comment
CVar = constraint variable
Func = Function
Pub = Public 
Priv = Private
Attr = Attribute
Prop = Property
Proc = Procedure
inv = invariant



asumptions:
 - define all applicable properties in abstract hfo class for inheritance purposes
 - constraint stuff is hidden for user (-> all constraints are defined by props)

hfo (CSS/SMIL inspired)
---
 pub Attr	: id, type
 pub Attr	: parent

 priv CVar	: x1,x2,y1,y2,t1,t2 % defines containing block = content + padding + margin
 priv CVar	: width % = width of content box (x2-x1-lm-lb-lp-rm-rb-rp)
 priv CVar	: height % = heigh of content box (x2-x1-lm-lb-lp-rm-rb-rp)
 priv CVar	: duration % = duration of content box (t2-t1-db-da)

 priv CVar	: margin-left|right|top|bottom % ml,mr,mt,mb (default:>=0)
 priv CVar	: padding-left|right|top|bottom % pl,pr,pt,pb(default:>=0) 
 priv CVar	: border-left|right|top|bottom-width % bl,br,bt,bb (default:>=0)
 priv CVar	: delayStart, delayEnd % ds,de (default:>=0)
 priv CVar	: delayBetween % db (default: >=0) - delay between children (so not before and after)

 Prop   : width*,min-width,max-width,fill-width % fill-width =min|max|center (default:min)
 Prop   : height*,min-height,max-height,fill-Height % fill-width =min|max|center (default:min)
 Prop   : duration*, min-duration, max-duration, fill-duration % = min|max|center (default:min)
 Prop   : min-pauseBefore, max-pauseBefore, fill-pauseBefore % = min|max|center (default:min)
 Prop   : min-pause, max-pause, fill-pause % = min|max|center (default:min)
 Prop   : min-pauseBetween, max-pauseBetween, fill-pauseBetween % = min|max|center (default:min)

	  (similar for margins,paddings,border)
	  m/l*, min-m/l,max-m/l,fill-m/l  % m/l = margin left

 * = syntactic sugar is re-writen to min-X:V max-X:V 

 Prop	: equal-margin,equal-padding,equal-border 

 Prop   : transIn, transOut % (default: inherit)
 Prop   : background % (default:inherit) background of content and padding (margin is transparent)
 Prop   : color,font,font-size,font-weight,font-family % (default:inherit)
 Prop   : border-left|right|top|bottom-color (default:inherit)
 Prop   : border-left|right|top|bottom-style (default:inherit)
 Prop   : horizontal-align (top,bottom,center,fill) % (default: inherit) apply's to children
 Prop   : vertical-align (left,right,center,fill) % (default: inherit) apply's to children
 Prop   : temporal-align (start,end,center,fill) % (default: inherit) apply's to children

 Func   : containerWidth % width of the whole box (x2-x1)
 Func   : containerheight % height of the whole box (y2-y1)
 Func   : containerDuration % duration of the whole box (y2-y1)

 Proc   : init(id,type)

	atomic
	------
	  Attr	: url|content 
	  Proc  : init(url|content)

		IMAGE
		-----
		  Proc	: init(width,height)

		AUDIO
		-----
		  Proc	: init(duration)

		TEXT
		-----
		  Proc	: init(textLength fontSize, fontType, textAlign)

	composite
	---------
  	  Attr	: children 
	  Proc	: init(Children)	

         	HFOROOT
                -------
		  Proc  : init(maxHeight,maxWidth,maxDur)
		  Prop	: bgmusic,bandWidth, etc. (deliveryContext)
		  inv	: #children = 1

		BOX 
		-----
		  Proc	: init(hbox|vbox|tbox)

			SLIDESHOW
			---------
			
			COLLAGE
			-------

		GRID
		-----
		  Proc	: init(#row,#column)