Including .XSL files into a master .XSL template

I am very new to .XSL and I am trying to understand how to include several =
external .XSL files into a master .XSL template.

The issue I am trying to solve is; I have 15+ templates and ~ 10 unique sec=
tions included in each template. A section is basically a data driven table=
 with a table header.   Now, each template can have any combination of the =
these sections.  As you can imagine, maintaining these files can be very te=
dious.  One change means I have to change 1 - 15 files.   Any advice on the=
 best way to handle this would be greatly appreciated.

My thoughts were

Master Template 1 - font style 1
                Section 1
                Section 2
                Section 3

Master Template 2 - font style 2
                Section 1
                Section 3
                Section 4
                Section 6

Master Template 3 - font style 1
                Section 3
                Section 4
                Section 5