Sub-texts, parts of original (ur-) texts.
The signature and semantics matches those of UStorage.
The type of original texts.
The type of indexes of original texts.
refer t i j returns the part of t from i until j.
The character pointed by j is not included in the result.
If j is equal to i or located before j, the result is
an empty string.
refer t i j
excerpt t copies the contents of t as a new ur_text.
context t returns the tuple (s, i, j) such that
t = refer s i j.
(s, i, j)
t = refer s i j
Conversion from indexes of sub-texts to ur_texts.