17.4 Controlling Compilation
  BASIC lets you control the compilation of a program by cre-
  ating and testing lexical constants. You create and assign
  values to lexical constants with the %LET directive. These
  constants are always LONG integers.

  You control the compilation by using the %IF-%THEN-
  %ELSE-%END %IF directive to test these lexical constants.
  Thus, you can conditionally:

  .
        Supply different values for program variables and con-
        stants
  .
        Skip over part of a program
  .
        Abort a compilation
  .
        Include BASIC source code from another file
  .
        Display informational messages during the compilation

  BASIC also supplies the lexical built-in function %VARIANT
  that can be used to conditionally control compilation.

  %IF-%THEN-%ELSE-%END %IF uses lexical expressions
  to determine whether to execute directives in the %THEN
  clause or the %ELSE clause. The following sections describe
  the use of:

  .
        Lexical constants and expressions (%LET directive)
  .
        %VARIANT
  .
        %ABORT
  .
        %PRINT
  .
        %IF-%THEN-%ELSE-%END %IF