In 'mount_hours()': emphasizes the initial condition that the tuple lessons be recorded separately from the other lessons (in 'TPL', NOT in 'LSS').
In 'mount_hours()': remove the inutil comment about the "global assignment" ("... will affect the environment of the function from which this internal function is called, NOT the global-environment").
In 'mount_hours()': we added a control over the resulting number of gaps (teachers should not get more than two gaps).
I decided to give up 'testthat' (I removed 'test-mount_hours.R', which took a long time); instead, I added the exported function 'verify_matrix()'.
I wrapped the examples in '\donttest{}'.
minor improvements to documentation.