Template Declarations - Why not Template definitions?

[WhatMeWorry via Digitalmars-d-learn]

I was reading "D Templates: A Tutorial" by Philippe Sigaud which 
says:

--------------------------- quote
What is a Template? In the next chapters, you’ll see how to 
define function, struct and class templates.

But before that, I’d like to introduce what a template really is, 
because this
definition is the most fundamental of the whole document. As I 
said, a template
is a way to define a blueprint to generate some code, be it a 
class definition, a
function or. . . what? What could be the most abstract unit of 
code?

But where is the basic unit to hold this code? Well, a code block 
of course, or a
scope.

This is what a template is, at its core: a named, parameterized,
code block, ready to be instantiated just for you.
--------------------------- end quote


so thinking that I had reached a first principle, I wrote the 
above template.
But then when this failed to compile, I then read further and it 
says:


--------------------------- quote
Template Declarations

Here is the syntax for a template declaration:

template templateName(list, of, parameters)
{
     // Some syntactically correct declarations here
     // The arguments are accessible inside the template scope.
}
--------------------------- end quote

so the code block must consist only of declarations.  Which lead 
to me to ask my original question.

mixins could work but I'm just trying to understand templates and 
this particular limitation.