Add Ordering#<=
Let Engine::Sort be a module.
Add `parse` in spec helpers.
Start a Compilable abstraction in Engine.
Don't use comma-separated inclusion list.
Add Operand#compile
Module conventions in Engine when mutliple strategies.
Add TypeCheck#heading.
Typo.
Complete Compilable abstraction.
Compilable#compile and sub-methods return a cog.
Merge Operand#compile and Operand#to_cog.
Add a frame example.
Add compilation of `frame` and ensure compilation is ok.
Remove Engine::Compiler for Operand#to_cog.
Factorize Relvar's common methods.
Use Operator#rubycase_name shortcut
Remove Relvar::Base#name
Clean Relvar::ReadOnly
Add cog and parser attr_readers in Engine::Compilable.