If length is omitted,
returns the character at position n
of the receiving string.
If n is larger than the length of the
receiving string, a null string is returned.
If length is specified,
returns the substring of the receiving string starting at position
n and of length length.
No padding occurs for any portion of the returned string not within the
boundaries of the receiving string.
The n must be a positive whole number,
and, if specified, length must be
a non-negative whole number.
