bool = IsAlpha(s$[, pos, encoding])
pos inside string s$ is an alphabetic letter and
returns True if it is, otherwise False. The optional pos parameter must be in
characters, not in bytes. It defaults to 0 which will test the first character
in s$.
The optional encoding parameter can be used to set the character encoding
to use. This defaults to the default string encoding set using SetDefaultEncoding().
See Character encodings for details.
True or False, depending on the test's result