The state of the linking check. It keeps track of compilation units provided and required so far.

create ~complete returns an empty state. If complete is true, missing compilation units will be treated as errors.

add t ~filename ~compunit ~provides ~requires registers the compilation unit compunit found in filename to t.

• provides are units and sub-units provided by compunit
• requires are units required by compunit

add should be called in reverse topological order.

required t compunit returns true if compunit is a dependency of previously added compilation units.

check t should be called once all the compilation units to be linked have been added. It returns some error if:

• There are some missing implementations and complete is true
• Some implementation appear before their dependencies