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function [b,sexp]=trisolve(a,b,sexp)
//[x [,sexp]] = trisolve(A,b [,sexp]) symbolically solves A*x =b
// when A and b are matrices of character strings, A being assumed to be
// upper triangular.
//sexp : vector of common subexpressions in A, b and x.
//!
//origine F. D. S. Steer INRIA 1989
//
// Copyright INRIA
[lhs,rhs]=argn(0)
[n0,m]=size(b)
if rhs==2 then
for n=n0:-1:1
pivot=a(n,n)
for k=1:m,b(n,k)=ldivf(pivot,b(n,k)),end
if n==1 then return,end
for l=1:n-1
for k=1:m,
b(l,k)=addf(b(l,k),mulf(mulf('-1',a(l,n)),b(n,k)))
end
end
end
else
ns=prod(size(sexp))
for n=n0:-1:1
pivot=a(n,n)
for k=1:m,
ns=ns+1
sexp(ns)=ldivf(pivot,b(n,k))
b(n,k)='%('+string(ns)+')';
end
if n==1 then return,end
for l=1:n-1
for k=1:m,
b(l,k)=addf(b(l,k),mulf(mulf('-1',a(l,n)),b(n,k)))
end
end
end
end
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