Side-Effects

[%  if add_header
        fill site/header
%]

[%  fill site/header
        if add_header
%]
--EOF--
look inside...

Thus Spake Andy:

All of TT3's block commands can be used in side-effect form (in fancy lingo, all commands are operators and block commands masquerade as prefix or infix operators). Here's an example using if...