A conditional assembly block is a series of source statements
    that are assembled only if a certain condition is met. A .IF
    starts the conditional block and a .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 .ENDC
    are assembled. If the test is not met, the statements are not
    assembled. Optionally, you can use the .ELSE directive (or a
    combination of the .IFF, .IFT, and .IFTF directives) to specify
    an alternate series of statements to assemble if the test is not
    met.

    You can nest conditional blocks; 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.