Add the first test. This test currently checks for empty graphs and for constant value mappings.
Add a getAllMappings method to the Projector which allows us to enter into the spi to check for the existence of mappings
Update the code to make the test pass and to satisfy the new method
Refactor the constant value tests to make them easier to add more tests to it
Report our first error. This will throw an exception if a mapping has no source. For example creating a graph with `map("/some/path");` will error
Change the name of the method that will map to a constant.