subroutine y12mde(n,a,nn,b,pivot,snr,ha,iha,iflag,ifail)
implicit real(a-b,g,p,t-y),integer (c,f,h-n,r-s,z)
real a(nn), pivot(n), b(n)
integer snr(nn), ha(iha,11), iflag(10)
if(iflag(1).eq.-2)go to 1000
ifail=1
go to 1110
ipiv=iflag(3)
n7=n-1
state=iflag(5)
c
c solve the system with lower triangular matrix l (if the
c lu-factorization is available).
c
if(state.ne.3)go to 1051
if(ipiv.eq.0)go to 1020
do 1010 i=1,n7
l1=ha(i,7)
t=b(l1)
b(l1)=b(i)
b(i)=t
do 1050 i=1,n
rr1=ha(i,1)
rr2=ha(i,2)-1
if(rr1.gt.rr2)go to 1040
do 1030 j=rr1,rr2
l1=snr(j)
1030 b(i)=b(i)-a(j)*b(l1)
1040 continue
1050 continue
c
c solve the system with upper triagular matrix.
c
1051 n8=n+1
do 1090 i=1,n
r1=n8-i
rr1=ha(r1,2)
rr2=ha(r1,3)
if(rr2.lt.rr1) go to 1080
do 1070 j=rr1,rr2
r2=snr(j)
1070 b(r1)=b(r1)-a(j)*b(r2)
1080 continue
1090 b(r1)=b(r1)/pivot(r1)
c
c if interchanges were used during the elimination then a reordering in
c the solution vector is made.
c
if(ipiv.eq.0)go to 1110
do 1100 i=1,n7
r1=n-i
r2=ha(r1,8)
t=b(r2)
b(r2)=b(r1)
1100 b(r1)=t
1110 return
end
.
