Used with `[..] and lfun::`[..] to specify how the
corresponding index maps to an upper or lower range bound:
INDEX_FROM_BEG
The index is relative to the beginning of the string or array
(or any other sequence implemented through an object).
Sequences typically start at zero.
INDEX_FROM_END
The index is relative to the end of the sequence. In strings
and arrays, the last element is at zero, the one before that
at one, etc.
OPEN_BOUND
The range is open in the corresponding direction. The index is
irrelevant in this case.