Add mechanisme to simplify constraint management for fluent syntax

[dupdob]

Some checks assume or require that some other check(s) has(ve) been done. Example: Once() or Which().
Current approach is to add full fledged dedicated type for those. This approach is cumbersome and ill fitted for custom extension.
A lighter mechanism should be devised.

Design remains to be done.

Created thanks to a discussion with @rhwy