o  If a macro has the same name as an Alpha opcode, the macro
       is used instead of the instruction. This feature allows you to
       temporarily redefine an opcode.

    o  You can redefine a macro by using a .MACRO directive with the
       same name as in a previous macro definition. Therefore, the
       previous macro definition is implicitly deleted and the new
       macro definition supersedes the previous definition. See the
       .MDELETE directive for more information on macro deletion.

    o  You can nest a macro definition within another macro
       definition. The inner macro is not defined until the outer
       macro is invoked.

    o  You can nest a macro invocation so that one macro can invoke
       another. The assembler supports nested macro invocations to a
       depth of 1000. If a macro invokes itself, either directly or
       indirectly, it is recursive. Recursive macros must specify a
       basis-step in order to avoid infinite recursion. A basis-step
       is a macro exit condition that will eventually cause the macro
       to exit, which ends the recursion (see Example 3).