instructions:
* B₀ = { x = 0, inc x }
* B₁ = B₀ { x = y }
* B₂ = B₀ { dec x }
* B₃ = B₁ B₂

For we define the loop language Li as the smallest set of programs generated by the following rules:

1. Every instruction s ? B_i is a program in Li.

2. If P and Q are in Li, then the sequential composition
:PQ