17.4.1 %LET Directive
  The %LET directive creates and assigns values to lexical
  constants. Lexical constants are always LONG integers.
  These constants control the execution of the %IF-%THEN-
  %ELSE-%END %IF directive.

  All lexical constants must be created with %LET before they
  can be used in a %IF-%THEN-%ELSE-%END %IF di-
  rective, and each lexical constant must be created with a
  separate %LET directive. All lexical constant names must
  also be preceded by a percent sign and cannot end with a
  dollar sign or percent sign.

  A lexical expression can be any of the following:

  .
        A lexical constant
  .
        An integer literal
  .
        A lexical built-in function (%VARIANT)
  .
        Any combination of these, separated by logical, relational,
        or arithmetic operators

  The %LET directive lets you create constants that control
  conditional compilation. For example:
  %LET %debug_on = 0%

  See Section 17.4.5 for an example of using %LET with %IF-
  %THEN-%ELSE.