`.if ABSOLUTE EXPRESSION'
=========================

   `.if' marks the beginning of a section of code which is only
considered part of the source program being assembled if the argument
(which must be an ABSOLUTE EXPRESSION) is non-zero.  The end of the
conditional section of code must be marked by `.endif' (see `.endif': Endif.); optionally, you may include code for the alternative
condition, flagged by `.else' (see `.else': Else..

   The following variants of `.if' are also supported:
`.ifdef SYMBOL'
     Assembles the following section of code if the specified SYMBOL
     has been defined.

`.ifndef SYMBOL'
`ifnotdef SYMBOL'
     Assembles the following section of code if the specified SYMBOL
     has not been defined.  Both spelling variants are equivalent.