Extracts a substring of length nCount
characters from the String, starting at position
nFirst. The function returns a copy of the extracted
substring. Mid is similar to the Basic MID$ function.
Arguments:
String - String from which the
middle part will be extracted.
nFirst - The position of the first
character in the String that is to be included in
the extracted substring.
nCount - The number of characters
to extract from String.
