For compatibility with VAX MACRO, MACRO-64 provides three
    directives for use within .IF blocks:

    Directive     Function

    .IF_FALSE     If the condition of the assembly block tests false,
                  the program includes the source code following
                  the .IF_FALSE directive and continuing up to the
                  next subconditional directive or to the end of the
                  conditional assembly block.
    .IF_TRUE      If the condition of the assembly block tests true,
                  the program includes the source code following
                  the .IF_TRUE directive and continuing up to the
                  next subconditional directive or to the end of the
                  conditional assembly block.
    .IF_TRUE_     Regardless of whether the condition of the
    FALSE         assembly block tests true or false, the source
                  code following the .IF TRUE_FALSE directive (and
                  continuing up to the next subconditional directive
                  or to the end of the assembly block) is always
                  included.

    The implied argument of a subconditional directive is the
    condition test specified when the conditional assembly block was
    entered. A conditional or subconditional directive in a nested
    conditional assembly block is not evaluated if the preceding (or
    outer) condition in the block is not satisfied (see Example 3 and
    Example 4).

    A conditional block with a subconditional directive is different
    from a nested conditional block. If the condition in the .IF is
    not met, the inner conditional blocks are not assembled, but a
    subconditional directive can cause a block to be assembled.