PROCEDURE mtt_implicit(VAR xnew,x,dx : StateVector;
		       VAR AA	     : StateMatrix;
		       VAR AAx	     : StateVector;
			   dt	     : REAL;
			   Nx	     : INTEGER;
		       VAR   open    : StateVector);

VAR
   i,ii,j,jj : INTEGER;
   BB,xsub   : StateVector;
   AAsub     : StateMatrix;
   
(*$I $MTTPATH/trans/p/mtt_solve.p *)

BEGIN{mtt_implicit}
   ii := 0;
   FOR i := 1 TO  Nx DO {BB is (1-A*dt)*x +dx*dt}
   BEGIN
      IF open[i]<0.5 THEN
      BEGIN
	 ii := ii+1; jj := 0;