SUBROUTINE DSOLVER(NL,GAMA,CP,CM,CPM1,CMM1
     ,,E1,E2,E3,E4,BTOP,BSURF,RSF,XK1,XK2)
C VERSION OF SOLVER
c     PARAMETER (NMAX=201)
      PARAMETER (NMAX=401)
      IMPLICIT REAL  (A-H,O-Z)
      DIMENSION GAMA(NL),CP(NL),CM(NL),
     ,CPM1(NL),CMM1(NL),XK1(NL),XK2(NL)
     ,,E1(NL),E2(NL),E3(NL),E4(NL)
      DIMENSION AF(NMAX),BF(NMAX),CF(NMAX),DF(NMAX),XK(NMAX)
C*********************************************************
C* THIS SUBROUTINE SOLVES FOR THE COEFFICIENTS OF THE    *
C* TWO STREAM SOLUTION FOR GENERAL BOUNDARY CONDITIONS   *
C* NO ASSUMPTION OF THE DEPENDENCE ON OPTICAL DEPTH OF   *
C* C-PLUS OR C-MINUS HAS BEEN MADE.                      *
C* NL     = NUMBER OF LAYERS IN THE MODEL                *
C* CP     = C-PLUS EVALUATED AT TAO=0 (TOP)              *
C* CM     = C-MINUS EVALUATED AT TAO=0 (TOP)             *
C* CPM1   = C-PLUS  EVALUATED AT TAOSTAR (BOTTOM)        *
C* CMM1   = C-MINUS EVALUATED AT TAOSTAR (BOTTOM)        *
C* EP     = EXP(LAMDA*DTAU)                              *
C* EM     = 1/EP                                         *
C* E1     = EP + GAMA *EM                                *
C* E2     = EP - GAMA *EM                                *
C* E3     = GAMA*EP + EM                                 *
C* E4     = GAMA*EP - EM                                 *
C* BTOP   = THE DIFFUSE RADIATION INTO THE MODEL AT TOP  *
C* BSURF  = THE DIFFUSE RADIATION INTO THE MODEL AT      *
C*          THE BOTTOM: INCLUDES EMMISION AND REFLECTION *
C*          OF THE UNATTENUATED PORTION OF THE DIRECT    *
C*          BEAM. BSTAR+RSF*FO*EXP(-TAOSTAR/U0)          *
C* RSF    = REFLECTIVITY OF THE SURFACE                  *
C* XK1    = COEFFICIENT OF THE POSITIVE EXP TERM         *
C* XK2    = COEFFICIENT OF THE NEGATIVE EXP TERM         *
C*********************************************************
      L=2*NL
C************MIXED COEFFICENTS**********
C* THIS VERSION AVOIDS SINGULARITIES ASSOC.
C* WIRH W0=0 BY SOLVING FOR XK1+XK2, AND XK1-XK2.
      AF(1)=0.0
      BF(1)=GAMA(1)+1.
      CF(1)=GAMA(1)-1.
      DF(1)=BTOP-CMM1(1)
      N=0
      LM2=L-2
C* EVEN TERMS
      DO 10 I=2,LM2,2
          N=N+1
          AF(I)=(E1(N)+E3(N))*(GAMA(N+1)-1.)
          BF(I)=(E2(N)+E4(N))*(GAMA(N+1)-1.)
          CF(I)=2.*(1.-GAMA(N+1)**2)
          DF(I)=(GAMA(N+1)-1.) * (CPM1(N+1) - CP(N))
     &          + (1.-GAMA(N+1))* (CM(N)-CMM1(N+1))
   10 CONTINUE
      N=0
      LM1=L-1
      DO 20 I=3,LM1,2
          N=N+1
          AF(I)=2.*(1.-GAMA(N)**2)
          BF(I)=(E1(N)-E3(N))*(1.+GAMA(N+1))
          CF(I)=(E1(N)+E3(N))*(GAMA(N+1)-1.)
          DF(I)=E3(N)*(CPM1(N+1) - CP(N))
     &         + E1(N)*(CM(N) - CMM1(N+1))
   20 CONTINUE
      AF(L)=E1(NL)-RSF*E3(NL)
      BF(L)=E2(NL)-RSF*E4(NL)
      CF(L)=0.0
      DF(L)=BSURF-CP(NL)+RSF*CM(NL)
      CALL DTRIDGL(L,AF,BF,CF,DF,XK)
C***UNMIX THE COEFFICIENTS****
      DO 28 N=1,NL
      XK1(N)=XK(2*N-1)+XK(2*N)
      XK2(N)=XK(2*N-1)-XK(2*N)
C NOW TEST TO SEE IF XK2 IS REALLY ZERO TO THE LIMIT OF THE
C MACHINE ACCURACY  = 1 .E -30
C XK2 IS THE COEFFICEINT OF THE GROWING EXPONENTIAL AND MUST
C BE TREATED CAREFULLY
      IF (XK2(N) .EQ. 0.0) GO TO 28
      IF (ABS (XK2(N)/XK(2*N-1)) .LT. 1.E-30) XK2(N)=0.0
   28 CONTINUE
      RETURN
      END