Behavioural RTL resembles software.
A behavioural thread assigns to variables, makes reference to variables already updated and can re-assign new values.
Unlike 'pure RTL', the order of the statements has an effect.
The following behavioural code
if (k) foo = y;
bar = !foo;
can be compiled down to the following, unordered 'pure RTL'.
foo <= (k) ? y: foo;
bar <= !((k) ? y: foo);
Not all behavioural RTL is defined to `synthesisable' (see later, and also synthesis algorithm in additional material).