documentation about constraints and backtracking

[Bruno Haible]

Hi,

The constraint system of poke is impressing. But the manual is silent about it.
I would find it good if in the "Structs" chapter, between the sections
"Union Constructors" and "Integral Unions", there was a section
"Constraints and Backtracking". This section would show three examples:

* Example float0.pk
  Highlights the importance that the alternative that can be empty comes last.

* Example float1.pk
  Shows how poke can distinguish alternative1 and alternative2. They differ
  in the first byte, but this first byte is hidden in nested structs.

* Example float2.pk
  Shows how poke can distinguish alternative1 and alternative2, although they
  differ not in the first byte, but only in the second byte.

You can put these three files in the pickles/ directory. But their value
really comes only with a documentation section.

Bruno

float0.pk
Description: application/tex-pk

float1.pk
Description: application/tex-pk

float2.pk
Description: application/tex-pk