An <EM>action</EM> is an ordinary Boolean-valued expression that may contain
primed and unprimed variables.  An action is true of a
 <A HREF = "step.html">step</A> 
 <CODE>&lt;s, t&gt;</CODE> 
iff the expression obtained by replacing
unprimed variables with their values in
   <A HREF = "states.html">state</A>
 <CODE>s</CODE>
and primed variables with their values in
   <A HREF = "states.html">state</A>
 <CODE>t</CODE>
is true.