The D language supports conditional compilation using debug and version blocks.
 They look like this:

    debug { ... }
    debug (xyz) { ... }
    debug (123) { ... }
    version (xyz) { ... }
    version (123) { ... }

The braces are optional - debug and version can also apply to a single statement
or declaration.  Moreover, any of these can also have 'else' blocks following.

Doxygen doesn't recognise this feature of D at present, and so contents of debug
or version blocks are not documented.

It will be necessary to provide a means in Doxygen of setting which
debug/version identifiers are conditioned in.  A start would be to map them by
some prefix to the space of preprocessor macros.  But better still would be to
add new tags to Doxygen for these.

Note also the special meaning of numeric debug/version identifiers - they are
compiled in iff a greater or equal version level is specified at compile time. 
This behaviour should carry forward to Doxygen.