<REXX> element reference
in your HEAD block. You can then use
the following construct within any other element within the BODY block:
<ANYTAG NAME="=("MyREXXCode att_param")" ...>
...
=("MyREXXCode other_param")
...
</ANYTAG>
which would be translated before it is interpreted into:
<ANYTAG NAME="ABCatt_paramXYZ" ...> ... ABCother_paramXYZ ... </ANYTAG>which doesn't make much sense, but it shows how it works in principle.
See "Sample script with REXX"
for a more meaningful example.