% Describe FMMD

\section{Background: FMMD outline}
The symptom abstraction process described here, is a core process in the 
%Failure Mode Modular De-Composition 
FMMD modelling technique. FMMD builds a hierarchy of failure mode behaviours from the bottom up.
To start with collections of base components are grouped into functional~groups, which are analysed
and then treated as components in thier own right. These higher level, or derived compoenents,
can be used to build derived components at higher levels of abstraction, and ultimately
are used to build an FMMD fault model hierarchy of the system modelled.
Intermediate FMMD structures may be re-used in other designs, and 
re-used in repeated sections of a design (consider a system
with several safety critical digital inputs for instance).