Returns:
True if any of the conditions described above is met.

Example:
The following example tests whether the routing information field in a Token Ring frame is null:

    @isnull(TOK:RI)

The following example tests whether the command field in an HTTP frame is not null:

    not(@isnull(HTTP:CMD.STR))

Tips:
This function can be very useful in conjunction with the not operator, if you wish to test whether a particular field is present and has contents, but you do not necessarily care what the contents of that field are.  If, however, you wish to test for the presence of a particular protocol in the current record, the @protocol() function would be a better choice.