[size=150][color=#000000][size=200]Aim: easy to use learning environment. [br]Define a [b]state [/b]A (variable) and the stock (start value of A) and define the [b]change rate[/b] v[sub]A[/sub] by a term. [br]New stock = old stock + change.[/size][br][size=200]It is the fundamental idea of [b]accumulation[/b]. Diagram:[/size][/color][/size]

[size=150][color=#000000][size=200]Blue cube: [b]Container[/b]. State A, variable A. [br]Green circle: [b]Valve[/b]. Change rate.[br]One state - one change rate![/size][br][/color][size=200][color=#000000]Step by step: Discrete calculation and plotting points.[/color] [/size][/size]