A conditional assembly block is a series of source statements
    that is assembled only if a certain condition is met. .IF starts
    the conditional block and .ENDC ends the conditional block; each
    .IF must have a corresponding .ENDC. The .IF directive contains
    a condition test and one or two arguments. The condition test
    specified is applied to the arguments. If the test is met, all
    MACRO-64 statements between .IF and .ELSE are assembled. If
    the test is not met, the statements between .ELSE and .ENDC are
    assembled.

    Conditional blocks can be nested; that is, a conditional block
    can be inside another conditional block. In this case, the
    statements in the inner conditional block are assembled only
    if the condition is met for both the outer and inner block. For
    more information, see the description of the .IF directive.