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This library provides generic functions for string manipulation,
such as finding and extracting substrings.
When indexing a string, the first character has position 1.
See Section 7.2 for some examples on string manipulation
in Lua.

Receives two string arguments,
and returns a number.
This number indicates the first position where the second argument appears
in the first argument.
If the second argument is not a substring of the first one,
then **strfind** returns **nil**.
A third optional numerical argument specifies where to start the search.
Another optional numerical argument specifies where to stop it.

Receives a string and returns its length.

Returns another string, which is a substring of **s**,
starting at **i** and runing until **j**.
If **j** is absent,
it is assumed to be equal to the length of **s**.
Particularly, the call **strsub(s,1,j)** returns a prefix of **s**
with length **j**,
while the call **strsub(s,i)** returns a suffix of **s**,
starting at **i**.

Receives a string and returns a copy of that string with all
upper case letters changed to lower case.
All other characters are left unchanged.

Receives a string and returns a copy of that string with all
lower case letters changed to upper case.
All other characters are left unchanged.

Returns the ascii code of the character **s[i]**.
If **i** is absent, it is assumed to be 1.

Receives 0 or more numbers.
Returns a string with length equal to the number of arguments,
wherein each character has ascii value equal
to its correspondent argument.

** Next:** 6.3 Mathematical Functions
**Up:** 6 Predefined Functions and
** Previous:** 6.1 Predefined Functions