Improve code coverage

[sesam]

I think this project has quite good test coverage already, from looking through some test files. Still it could be nice with a ticket find the current coverage, and to figure out together if some more tests should be added.

[blueworrybear]

Hi @sesam,

I am interested in this issue. I fork your repository and generate the coverage report. Here is the result:

It seems to me that we could have more test cases in the runtime package, especial for reference.go and visitor.go.

May I work on this issue? Thank you and happy hacktoberfest!