Sequences may not include ranges that go from a larger value to a smaller, e.g., 4 - 2.
Affects: @pos
@chars
@seg
@feature-qty-test
Used by template ŧ insert-seg-into-leaf-divs-in-hierarchy-fragment
ŧ prep-tan-mor
Used by function tan:get-toks
() tan:sequence-error
()