12.2 Using Dynamic Strings
  Although dynamic strings are less efficient than fixed-length
  strings, they are often more flexible. For example, to con-
  catenate strings, you can use the LET statement to assign
  the concatenated value to a dynamic string variable, without
  having to be concerned about BASIC truncating the string
  or adding trailing spaces to it. However, if the destination
  variable is fixed-length, you must make sure that it is long
  enough to receive the concatenated string, or BASIC trun-
  cates the new value to fit the destination string. Similarly, if
  you use LSET or RSET to concatenate strings, you must en-
  sure that the destination variable is long enough to receive
  the data.

  The LET, LSET, and RSET statements all operate on dy-
  namic strings as well as fixed-length strings. The LET
  statement can change the length of a dynamic string; LSET
  and RSET do not. LSET and RSET are more efficient than
  LET when changing the value of a dynamic string. For more
  information about LSET and RSET, see Sections 12.5.2 and
  12.5.3.

  In the following example, the first line assigns the value
  ``ABC'' to A$ , the second line assigns ``XYZ'' to B$ , and the
  third line assigns six spaces to C$ . These variables are dy-
  namic strings. In the fourth line, LSET assigns A$ the value
  of A$ concatenated with B$ . Because the LSET statement
  does not change the length of the destination string vari-
  able, only the first three characters of the expression A$ + B$
  are assigned to A$ . The fifth line uses LSET to assign C$ the
  value of A$ concatenated with B$ . Because C$ already has a
  length of 6, this statement assigns the value ``ABCXYZ'' to it.
  LET A$ = "ABC"
  LET B$ = "XYZ"
  LET C$ = " "
  LSET A$ = A$ + B$
  LSET C$ = A$ + B$
  PRINT A$
  PRINT C$
  END

  Output
  ABC
  ABCXYZ
  Like the LET statement, the INPUT, INPUT LINE, and
  LINPUT statements can change the length of a dynamic
  string, but they cannot change the length of a fixed-length
  string.

  In this example, the first line assigns the null string to vari-
  able A$ . The second line uses the LEN function to show
  that the null string has a length of zero. The third line uses
  the INPUT statement to assign a new value to A$ , and the
  fourth and fifth lines print the new value and its length.
  !Declare a dynamic string
  LET A$ = ""
  PRINT LEN(A$)
  INPUT A$
  PRINT A$
  PRINT LEN(A$)
  END

  Output
    0
  ? THIS IS A TEST
  THIS IS A TEST
    14
  You should not confuse the null string with a null character.
  A null character is one whose ASCII numeric code is zero.
  The null string is a string whose length is zero.