Mid$ (string, offset)
Mid$ (string, offset, length)

   Synopsis:
      Returns a string within a string

   Notes:
      If length is 0, an empty string is returned. If offset plus length is
         beyond the end of string, or if length is omitted, the remaining length
         of the string is used. If offset is less than 1, or if length is less
         than 0, an error is generated.

   Examples:
      ! Return everything in the string from the 6th character onwards
      Mid$ ("Read my lips", 6)
      ! Return 2 characters in a string, starting from the 6th character
      Mid$ ("Read my lips", 6, 2)
