coverage test on rel package

[nofun97]

Purpose

rel package coverage is very low. This is because most of the test is in the syntax package which depends on the rel package.

Suggested approaches

There's a feature in go where test files can have a different package name by adding _test as a suffix. e.g. package name_test.

This would allow independent import to the rel package and in turn might allow coverage to the rel package through the tests in syntax package.