Functional Library Sublayer
Principle:
The functional library layer's responsibility lies with exposing an interface/DSL through which the SUT can be driven on a higher, functional level.
This is the layer that the test scripts (and thus, the test engineer writing these test scripts) will use to drive the SUT.
It is therefor important to keep this interface as clean as possible, even more so than the other interfaces.
As a general rule. when you have to choose between simplifying this interface or another, the readability of the functional interface should take priority.
The combined interface for the functional and utility layers become a human-readable DSL for driving the SUT.
Understanding the intent of the test scripts should be clear already from reading the code, so design for readability and verbosity.