def mark(l, h, idx):
return [(l, f"<STR_LIT>"), (h + <NUM_LIT:1>, f"<STR_LIT>")]<EOL>
Produce markers based on argument positions

:param l: sentence position of first word in argument.
:type l: int
:param h: sentence position of last word in argument.
:type h: int
:param idx: argument index (1 or 2).
:type idx: int
:return: markers.
:rtype: list of markers