dOWI1, and left constitute an incomplete set of arrow-cursor movement tasks,
because right is missing. Once the full set of task-action mappings is written
down, the grammar of the command language can be tested against it to
demonstrate completeness. Of course, a designer might leave out something
from the task-action mapping, such that the grammar could not be checked
accurately, but it still seems useful to have an approach to checking for completeness and consistency. For example, a TAG definition of cursor control
would have a dictionary of tasks: