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diff --git a/Trivac/src/NSS4TR.f b/Trivac/src/NSS4TR.f
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+*DECK NSS4TR
+      SUBROUTINE NSS4TR(NEL,NMIX,MAT,XX,IQFR,QFR,DIFF,SIGR,FD,A11)
+*
+*-----------------------------------------------------------------------
+*
+*Purpose:
+* Assembly of leakage system matrices for the coarse mesh finite
+* difference method.
+*
+*Copyright:
+* Copyright (C) 2021 Ecole Polytechnique de Montreal
+* This library is free software; you can redistribute it and/or
+* modify it under the terms of the GNU Lesser General Public
+* License as published by the Free Software Foundation; either
+* version 2.1 of the License, or (at your option) any later version
+*
+*Author(s): A. Hebert
+*
+*Parameters: input
+* NEL     number of nodes
+* NMIX    number of mixtures
+* MAT     node mixtures
+* XX      node widths
+* IQFR    boundary conditions
+* QFR     albedo functions
+* DIFF    diffusion coefficients
+* SIGR    macroscopic removal cross sections
+* FD      discontinuity factors
+*
+*Parameters: output
+* A11     assembly matrix.
+*
+*-----------------------------------------------------------------------
+*
+      INTEGER NEL,NMIX,MAT(NEL),IQFR(6,NEL)
+      REAL XX(NEL),QFR(6,NEL),DIFF(NMIX),SIGR(NMIX),FD(NMIX,2),
+     1 A11(3*NEL,3*NEL)
+*
+      A11(:3*NEL,:3*NEL)=0.0
+      ! WEIGHT RESIDUAL EQUATIONS:
+      NUM1=0
+      DO KEL=1,NEL
+        IBM=MAT(KEL)
+        DX2=XX(KEL)**2
+        SIGG=SIGR(IBM)
+        DIDD=DIFF(IBM)
+        A11(NUM1+1,NUM1+1)=SIGG
+        A11(NUM1+1,NUM1+3)=-2.0*DIDD/DX2
+        A11(NUM1+2,NUM1+2)=SIGG/12.0
+        A11(NUM1+3,NUM1+3)=SIGG/180.0
+        NUM1=NUM1+3
+      ENDDO
+      ! continuity relations:
+      NUM1=0
+      DO KEL=1,NEL-1
+        IBM=MAT(KEL)
+        IBMP=MAT(KEL+1)
+        DIDD=DIFF(IBM)
+        DIDDP=DIFF(IBMP)
+        NUM2=NUM1+3
+        ! flux continuity:
+        FDP=FD(IBM,2)
+        FDM=FD(IBMP,1)
+        A11(NUM1+2,NUM1+1)=FDP
+        A11(NUM1+2,NUM1+2)=FDP/2.0
+        A11(NUM1+2,NUM1+3)=FDP/6.0
+        A11(NUM1+2,NUM2+1)=-FDM
+        A11(NUM1+2,NUM2+2)=FDM/2.0
+        A11(NUM1+2,NUM2+3)=-FDM/6.0
+        ! current contunuity:
+        A11(NUM1+3,NUM1+2)=DIDD/XX(KEL)
+        A11(NUM1+3,NUM1+3)=DIDD/XX(KEL)
+        A11(NUM1+3,NUM2+2)=-DIDDP/XX(KEL+1)
+        A11(NUM1+3,NUM2+3)=DIDDP/XX(KEL+1)
+        NUM1=NUM1+3
+      ENDDO
+      ! left boundary condition:
+      IBM=MAT(1)
+      IF((IQFR(1,1) == -1).OR.(IQFR(1,1) > 0)) THEN
+        ! VOID
+        AFACTOR=QFR(1,1)
+        A11(NUM1+2,1)=AFACTOR
+        A11(NUM1+2,2)=-(AFACTOR/2.0+DIFF(IBM)/XX(1))
+        A11(NUM1+2,3)=(AFACTOR/6.0+DIFF(IBM)/XX(1))
+      ELSE IF(IQFR(1,1) == -2) THEN
+        ! REFL
+        A11(NUM1+2,2)=1.0
+        A11(NUM1+2,3)=-1.0
+      ELSE IF(IQFR(1,1) == -3) THEN
+        ! ZERO
+        A11(NUM1+2,1)=1.0
+        A11(NUM1+2,2)=-1.0/2.0
+        A11(NUM1+2,3)=1.0/6.0
+      ENDIF
+      ! right boundary condition:
+      IBM=MAT(NEL)
+      IF((IQFR(2,NEL) == -1).OR.(IQFR(2,NEL) > 0)) THEN
+        NUM2=3*(NEL-1)
+        ! VOID
+        AFACTOR=QFR(2,NEL)
+        A11(NUM1+3,NUM2+1)=AFACTOR
+        A11(NUM1+3,NUM2+2)=(AFACTOR/2.0+DIFF(IBM)/XX(NEL))
+        A11(NUM1+3,NUM2+3)=(AFACTOR/6.0+DIFF(IBM)/XX(NEL))
+      ELSE IF(IQFR(2,NEL) == -2) THEN
+        NUM2=3*(NEL-1)
+        ! REFL
+        A11(NUM1+3,NUM2+2)=1.0
+        A11(NUM1+3,NUM2+3)=1.0
+      ELSE IF(IQFR(2,NEL) == -3) THEN
+        NUM2=3*(NEL-1)
+        ! ZERO
+        A11(NUM1+3,NUM2+1)=1.0
+        A11(NUM1+3,NUM2+2)=1.0/2.0
+        A11(NUM1+3,NUM2+3)=1.0/6.0
+      ENDIF
+      END SUBROUTINE NSS4TR