12.1 Overview of Strings
  A string is a sequence of ASCII characters. BASIC allows
  you to use the following types of strings:

  .
        Dynamic strings
  .
        Fixed-length strings
  .
        String virtual arrays

  Dynamic strings are strings whose length can change dur-
  ing program execution. The length of a dynamic string
  variable may or may not change, depending on the statement
  used to modify it.

  Fixed-length strings are strings whose length never
  changes. In other words, their length remains static. String
  constants are always fixed-length. String variables can be
  either fixed-length or dynamic. A string variable is fixed-
  length if it is named in a COMMON, MAP, or RECORD
  statement. If a string variable is not part of a map or com-
  mon block, RECORD, or virtual array, it is a dynamic string.
  When a string variable is fixed-length, its length does not
  change, regardless of the statement you use to modify it.
  Table 12-1 provides more information about string modifica-
  tion.

  Strings in virtual arrays have both fixed-length and dynamic
  attributes. String virtual arrays have a specified maximum
  length from 0 to 512 characters. During program execution,
  the length of an element in a string virtual array can change;
  however, the length is always from 0 to the maximum string
  size specified when the array was created. See Section 12.4
  and Chapter 14 for more information about virtual arrays.