metal:define-slot syntax:
argument ::= Name
The metal:define-slot statement defines a macro customization
point or slot. When a macro is used, its slots can be replaced,
in order to customize the macro. Slot definitions provide default
content for the slot. You will get the default slot contents if
you decide not to customize the macro when using it.
The metal:define-slot statement must be used inside a
metal:define-macro statement.
Slot names must be unique within a macro.
Simple macro with slot:
<p metal:define-macro="hello">
Hello <b metal:define-slot="name">World</b>
</p>
This example defines a macro with one slot named name. When you
use this macro you can customize the b element by filling the
name slot.