metal:fill-slot syntax:
argument ::= Name
The metal:fill-slot statement customizes a macro by replacing a
slot in the macro with the statement element (and its content).
The metal:fill-slot statement must be used inside a
metal:use-macro statement.
Slot names must be unique within a macro.
If the named slot does not exist within the macro, the slot contents will be silently dropped.
Given this macro:
<p metal:define-macro="hello">
Hello <b metal:define-slot="name">World</b>
</p>
You can fill the name slot like so:
<p metal:use-macro="container/master.html/macros/hello">
Hello <b metal:fill-slot="name">Kevin Bacon</b>
</p>