\define testmacro(one)
Result: $one$.
\end testmacro

<<testmacro one={{{ [[There]addprefix[Hello]] }}}>>

That renders as:

Result: HelloThere.

\function testfunction(one)
[<one>addprefix[Hello]]
\end testfunction

<<testfunction one={{{ [[re]addprefix[The]] }}}>>

That renders as:

HelloThere

\define testmacro(one)
Result: $one$.
\end testmacro

<$text text=<<testmacro one={{{ [[There]addprefix[Hello]] }}}>>/>

That renders as:

\function testfunction(one)
[<one>addprefix[Hello]]
\end testfunction

<$text text=<<testfunction one={{{ [[re]addprefix[The]] }}}>>/>

That renders as:

\define innermacro(one)
:$one$:$one$:
\end innermacro

\define mymacro(param)
|$param$|$param$|
\end mymacro

<div class=<<mymacro param={{{ [<innermacro one={{$:/palette}}>addprefix[The palette named ]] }}}>>>
Content
</div>

That renders as:

... and the underlying HTML is:

<p><div class="|The palette named :one={{$:/palette}}:one={{$:/palette}}:|The palette named :one={{$:/palette}}:one={{$:/palette}}:|">
Content
</div>
</p>