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  Directive:  #if, #elif, #else, #endif
 
  Syntax:     #if test expression
                  [text-block]
              [#elif test expression
                  text-block]
              ∙ ∙ ∙
              [#else
                  text-block]
              #endif
 
  Summary:    Checks each test expression associated with an #if or
              #elif directive until a true (nonzero) expression is
              found, then processes the <text-block> associated with
              that test expression. If there is an #else clause, the
              <text-block> associated with it is processed only if no
              test expression in the #if or #elif clauses has a nonzero
              value.
 
              The test expression can be any expression that evaluates
              to a constant, and can contain logical operators and the
              "defined" operator. It cannot use the sizeof operator,
              type casts, or the float or enum types. The <text-block>
              can contain C code or compiler directives.
 
  See also:   #define, defined, #ifdef, #ifndef, #undef
                                    -♦-