@m: Define FWEB macro, and mark

This command begins the definition part.

@m defines an FWEB macro, and also marks the next identifier as defined here. It is equivalent to

@m == @M@[

(see AT[).

For a detailed discussion of FWEB macros, see Macros.

The distinction between @m and @M is analagous to the distinction between @a and @A. See ATa.